CHAPTER III

PRELIMINARY REMARKS ON IMMUNITY IN THE ANIMAL KINGDOM

Examples of natural immunity among the Invertebrates.—Immunity against micro-organisms and insusceptibility to microbial poisons are two distinct properties.—The refractory organism does not eliminate micro-organisms by the excretory channels.—It destroys them by a process of resorption.—The fate of foreign bodies in the organism.—The resorption of cells.—Intracellular digestion.—This digestion effected by the aid of soluble ferments.—Digestion in Planarians and Actinians.—Actinodiastase.—Transition from intracellular digestion to digestion by secreted juices.—Digestion in the higher animals.—Enterokynase and the part it plays in digestion.—The psychical and nervous elements in digestion.—Adaptation of the pancreatic secretion to the kind of food.—Excretion of pepsin in the blood and in the urine.

[Sidenote: [43]]

As shown in the two preceding chapters unicellular organisms and plants afford evidence of numerous phenomena of immunity. Alongside natural immunity we find in them undoubted evidence of an adaptation to the presence of morbific agents, evidence which warrants us in inferring that cases of acquired immunity are frequent. This being the case it is quite natural that the animal kingdom should be no exception to the general rule. Indeed, immunity against pathogenic agents is widely distributed in animals, and we continually see manifestations of natural immunity not only against parasites and their toxins, but against poisons in general. Just as frequently we find cases of acquired immunity against these morbific agents.

[Sidenote: [44]]

As yet we know but little concerning the phenomena of immunity in the lower animals belonging to the great group of the Invertebrata. But it may be affirmed with certainty that these also are often endowed with a natural immunity against micro-organisms and bacterial toxins. As an example I may cite the case of the large white larvae of the Rhinoceros beetle (_Oryctes nasicornis_) frequently met with in tanner’s bark. Very susceptible to the cholera vibrio—¹⁄₈₀₀₀ of a culture[53] of this organism being sufficient to set up a fatal septicaemia—these larvae exhibit a very remarkable natural immunity against the bacilli of anthrax and diphtheria. A large dose of bacteria of the second anthrax vaccine, fatal to rabbits, guinea-pigs and mice, is borne without any inconvenience by the larvae of the Rhinoceros beetle. They are equally refractory to large doses of the diphtheria bacillus. And yet, there are not wanting species of insects which are susceptible to these same micro-organisms. Thus, according to A. Kovalevsky[54], crickets contract anthrax very readily even at moderate temperatures (22°–23° C.). On the other hand they are, according to the same author, refractory to the bacillus of avian tuberculosis. Many of the Invertebrata, studied from this point of view, present analogous facts, with which, however, we need not at present occupy ourselves.

In the Vertebrata in general and in Man in particular, natural immunity against many infective diseases and soluble poisons is so widespread that we are at no loss to find examples for citation. We have a whole series of human infections whose study is rendered particularly difficult simply because of the natural immunity of all other species of animals from these infections. Such are syphilis, scarlatina, leprosy, exanthematous typhus, etc. On the other hand, a large number of diseases, very infective for domestic animals, are quite innocuous to man. In this group we have cattle plague, strangles, contagious pleuro-pneumonia, fowl cholera, pneumo-enteritis of pigs, and a number of other diseases.

As in a very large majority of instances pathogenic organisms act through the agency of their toxic products, one might believe—and this has been assumed repeatedly—that natural immunity against infective diseases is dependent on the insusceptibility of the refractory organism to the specific poisons.

[Sidenote: [45]]

Such a supposition cannot survive criticism. We have undoubted instances of a species of animal being resistant both to a micro-organism and to its toxin. Such instances, however, are rare and usually an organism that is refractory or only slightly susceptible to the micro-organism itself is very susceptible to its toxic products. Even those micro-organisms which come almost constantly in contact with the human organism without becoming pathogenic, may produce toxins capable of gravely affecting health. Let us take as an example the bacillus of blue pus. This organism is most widely diffused in human surroundings. According to Schimmelbusch[55] it is met with on the skin of the arm-pits and of the inguinal region of one-half of mankind. From the skin it very often passes into the dressings of wounds which then assume the characteristic and so long recognised blue colour. The same bacillus is also found in the intestines of both sick and healthy persons. Jakowski[56] has met with it in the faeces coming from intestinal fistulae in two women who had undergone operations. Now, in spite of these specially favourable conditions for the production of infection, the _Bacillus pyocyaneus_ has remained harmless. It is only in children, and even then rarely, that it can be convicted of exciting disease. Man, then, usually enjoys a true natural immunity against the _Bacillus pyocyaneus_. And yet it is not to his insusceptibility to the pyocyanic toxin that he is indebted for this immunity. Schaffer[57], having injected himself in the shoulder with half a c.c. of a sterilised culture of _B. pyocyaneus_, developed fever and an erysipelatous swelling. Bouchard and Charrin[58] injected pyocyanic toxin into patients who reacted with more or less fever and by other toxic symptoms.

[Sidenote: [46]]

Another extremely common saprophyte, the _Micrococcus prodigiosus_, is incapable of setting up an infective disease, but this does not prevent its products from exercising a toxic action, often very grave, in man. The frog, which is refractory to the cholera vibrio, undergoes a fatal intoxication when cholera toxin is injected. One of the most striking examples is furnished in the case of the human tubercle bacillus and tuberculin. Man is much more resistant than is the guinea-pig to the pathogenic action of this organism, yet he is incomparably more susceptible to its toxin (tuberculin). According to the researches of Behring and Kitashima[59], the sheep, of all species of mammals, is most susceptible to the tubercular poison; the Bovidae and the guinea-pig occupy an inferior rank in the scale of susceptibility. On the other hand, the guinea-pig is very susceptible to the tubercle bacillus; the Bovidae are less so and the sheep is still more resistant to tuberculosis. It is unnecessary to multiply instances. Immunity against microbial infection and against intoxication are two distinct properties, so that it is impossible to reduce the former to an insusceptibility to toxins. We must therefore consider these two kinds of immunity separately and we will first consider the resistance of the animal organism against living infective micro-organisms.

Refractory human beings and animals may be inoculated with a large number of micro-organisms without being affected. Thus Opitz[60] injected 10,000,000 organisms into the blood of a dog. Twenty minutes later he could find no more than 9000. It is then quite natural to ask, What becomes of these micro-organisms after they have made their way into the interior of the refractory organism? It has been suggested that the animal gets rid of the pathogenic germs much as it does of all kinds of soluble poisons. Certain of these poisons, such as iodine and alcohol, are in great part eliminated by the kidneys; others, such as iron, by the alimentary canal. Why, it is asked, should not micro-organisms also be eliminated by the same channels? Flügge has adopted this view and has expounded it in his work on ferments and micro-organisms[61]. Moreover he suggested to Wyssokowitch[62] that he should carry out a large series of experiments with the object of verifying this theory. But numerous very careful researches have given a result quite at variance with the forecast made by Flügge. Micro-organisms of various species, injected into the blood vessels of rabbits and dogs, were, in those cases where these animals are refractory, never eliminated, either by the kidneys or by any other of the excretory channels which were studied. When bacteria pass into the secretions, lesions of the tissues, more or less grave, are invariably present.

[Sidenote: [47]]

This result has been repeatedly confirmed and has been accepted as a general experience. The elimination of micro-organisms by the urine indicates not merely the absence of immunity, but implies, also, a susceptibility of the organism. In many septicaemias, such as those produced by the anthrax bacillus, the streptococcus and other bacteria, or in less generalised diseases, such as typhoid fever, bacteria are found in the urine, often in large numbers. In these cases it is a question of anything but a refractory condition even of the slightest degree.

In recent years, however, several works have been published the aim of which was to demonstrate the inaccuracy of this apparently well-established thesis. Biedl and Kraus[63] in Vienna took the initiative and announced in a detailed work that micro-organisms can readily pass intact into the kidney and that this organ in virtue of its physiological function eliminates them. The organisms were said to leave the blood capillaries by the normal process of diapedesis and were then eliminated with the urine. The liver in a physiological condition, according to the researches of these authors, is equally capable of allowing of the passage of micro-organisms; indeed it aids in discharging them from the system. On the other hand, the pancreas and the salivary glands were incapable of fulfilling this function. Von Klecki[64] obtained similar results. He also holds that the kidney is the principal organ of elimination for micro-organisms which have penetrated into a refractory organism.

With these contradictions before him, Opitz[65] set himself to study this question in Flügge’s laboratory at Breslau. Having critically reviewed the technical methods of his predecessors and carried out a series of new experiments, he declared categorically “that a physiological excretion, by the kidneys, of the micro-organisms which circulate in the blood, does not exist.” For Opitz “the frequent appearance of micro-organisms in the urine of animals into whose blood, a short time previously, living bacteria have been injected, is due to mechanical and chemical lesions of the vessel wall and of the renal epithelia.”

[Sidenote: [48]]

This question might be looked upon as definitely settled in favour of the first results obtained by Wyssokowitch were it not that other voices had been raised in favour of a physiological excretion of the micro-organisms by the renal channels. Pawlowsky[66] has recently published a long work on this subject in which he attempts to demonstrate that certain micro-organisms, even when introduced into the subcutaneous tissue of animals, pass very rapidly (at the end of a quarter of an hour) into the uropoietic organs and are eliminated with the urine.

It was necessary to put an end to these controversies and Métin[67] undertook a series of researches at the Pasteur Institute with the object of clearing up this question. He guarded himself against the objections justly made against his predecessors and conducted his experiments under unexceptionable conditions. He injected several species of micro-organisms into the veins of rabbits and into the subcutaneous tissue of guinea-pigs. At various intervals he performed laparotomy on these animals, pulled out the bladder and drew off the urine in such a fashion that no trace of blood could get into it. The results were most conclusive. Never, when the experiment was conducted under the rigorous conditions just mentioned, did the micro-organisms traverse the kidneys of resistant animals nor were they ever met with in their urine.

Métin’s researches on the passage of micro-organisms through the liver in refractory animals gave the same results. In no case was he able to find in the bile any of the organisms that had been injected into the blood or under the skin. At the end of his memoir Métin sums up his results as follows: “(1) The kidneys and the liver are impermeable to bacteria introduced into the organism, subcutaneously or intravenously; (2) when the culture tubes contain colonies of the injected micro-organism, it is because there has been a certain amount of blood in the fluid inoculated, this being an indication of a vascular or epithelial lesion, either mechanical or chemical.” We were present at M. Métin’s experiments and can bear witness to their exactitude.

[Sidenote: [49]]

There can no longer be any doubt then on this point. The elimination of the micro-organisms from the refractory animal takes place, as indicated in Wyssokowitch’s first investigation, neither by the kidneys nor by the liver. Some observers have asserted that this elimination may take place by the sudoriparous glands. Thus, Brunner[68] made experiments with young pigs and cats into which he had previously injected micro-organisms, for the most part pathogenic. Then producing a transpiration by means of pilocarpin, he “cultivated” the sweat and noted the development of the same bacteria as he had introduced into the blood. In a single experiment with a saprophyte (_Coccobacillus prodigiosus_) he obtained a positive result, from which he concludes that the refractory animal gets rid of bacteria which circulate in its blood by way of the sudoriparous glands. It is scarcely allowable to draw any conclusion from this experiment from the fact that the snout of the pig, the seat of the transpiration, is very liable to small vascular lesions which might furnish the bacteria that developed on Brunner’s plates. Nevertheless, even in the case of pathogenic organisms, which swarm in the blood, the sweat is usually free from them. This has been shown by Krikliwy[69] in the case of cats inoculated with anthrax whose sweat, in spite of the passage of numerous bacteria into the circulation, contained none.

[Sidenote: [50]]

Micro-organisms, then, after their entrance into the refractory animal, are not eliminated by any of the excretory channels which serve for the elimination of many of the soluble poisons. It was necessary therefore to seek some other process capable of affording an explanation of the disappearance of the micro-organisms which so often and by such varied means make their way into the interior of a resistant organism. For it is a well-established fact that in these cases the micro-organisms do disappear completely. This has been observed so often that it is unnecessary to offer any demonstration of the fact. Perhaps in the refractory organism the micro-organisms undergo the fate of the foreign bodies which penetrate, or which are introduced, into the circulation. It has long been known, thanks especially to the work of Hoffmann and Recklinghausen[70], and of Ponfick[71], that particles of carmine or vermilion when injected into the blood are deposited in several organs. They are found in the spleen, the lymphatic glands and the bone-marrow. A certain number of these foreign particles may even be fixed in the liver and kidneys, but, instead of passing into the bile and the urine, they remain lodged in the interstitial tissue of the organs. The observers just cited noted that the coloured granules do not remain long in either the blood or the lymph but will be found in the interior of the cellular elements. These granules persist for weeks without any appreciable modification, differing in this from the micro-organisms which, as a rule, after several days or even after a few hours, disappear from the refractory organism. This disappearance might be more justly compared to the resorption of corpuscular elements which results in a more or less complete atrophy. The facts concerning the resorption of pus, of extravasated blood, of the mucosa of the uterus in pregnancy, etc., have long been known, and it is among these that one should seek analogies with the disappearance of the micro-organisms. When bacteria of various species are injected into refractory or not very susceptible animals, we always observe a local reaction in the form of inflammation, accompanied by the appearance of white corpuscles. Gradually the organisms disappear from the point at which they are introduced; the exudation becomes sterile and ultimately is completely absorbed. Numerous researches, which will be set forth in the succeeding chapters, have, indeed, demonstrated the remarkable analogy that exists between the disappearance of the micro-organisms from the refractory animal and the resorption of corpuscular elements or of animal cells.

The analysis of the phenomena of this resorption will help us considerably in our study of immunity against micro-organisms. When in any part of the animal organism a collection of pus, an effusion of blood, or any other organic lesion is produced, these lesions are usually repaired after the lapse of a longer or shorter interval. In those cases where the cells retain their integrity, they are taken into the lymphatic vessels and then pass into the circulating blood. In the course of his researches on the transfusion of blood, Hayem[72] observed “that blood injected into the peritoneum is absorbed unaltered and passes with its anatomical elements into the general circulation.” He was able to demonstrate “that the lymphatic channels play an important part in this absorption.” Lesage of Alfort[73] confirmed this result. He found that in the dog “one hour after an abundant haemorrhage into the peritoneum, induced experimentally, the red corpuscles commenced to pass freely, without alteration and in very large numbers, into the thoracic duct.” I have observed a similar resorption of the red blood corpuscles of the guinea-pig when injected into the peritoneal cavity of other individuals of the same species. The white corpuscles can also be taken up by the lymphatic vessels without being modified in any way. At the end of an inflammatory reaction of feeble intensity, set up in cold-blooded animals, especially in the tadpole, the direct passage of leucocytes from the exudation into the lymphatic system may be observed.

[Sidenote: [51]]

The examples I have just cited are, however, quite exceptional. In the great majority of cases the cellular elements that are undergoing resorption are seized by the amoeboid cells and are taken into their substance. Even in the resorption of the red corpuscles, lying free in the peritoneal cavity of the same species of animal, a certain number of the globules do not pass directly into the circulation but are first ingested by the amoeboid elements. This fact is insisted upon by Lesage. In inflammatory exudations the leucocytes also become the prey of their fellows. The ingested white corpuscles may be recognised for some time lying in the interior of other leucocytes; they are soon broken up, however, and finally disappear completely. When, instead of isolated cells such as leucocytes, we introduce fragments of tissues or of organs into any part of the organism, the same mode of resorption may always be observed. The introduced fragments are first surrounded and infiltrated by amoeboid cells and are then taken up into their interior.

[Sidenote: [52]]

The mode of absorption just described is very general. It applies to all kinds of cells and is observed in the absolutely normal organism, as well as in a large number of pathological conditions. For more than fifty years, the existence of cells which contain red blood corpuscles (“blutkörperchenhaltige Zellen” of German writers) has been recognised; they were met with in the spleen, the lymphatic glands and in many pathological products. For long we could not explain how the red corpuscles come to be inside other cells. Virchow[74] thought that they got there as the result of a mechanical pressure. Later histologists succeeded in determining the true nature of cells containing red blood corpuscles and in recognising that the leucocytes had really ingested the corpuscles. There has been much discussion, also, on the presence of leucocytes in the interior of large cells in exudations. It was thought that these were mother-cells which contained a new generation of small cells. Writers even described a fusion between the large cell and those found inside it; but Bizzozero[75] first recognised that the former was an amoeboid cell which had ingested pus corpuscles. Since this observation was made numerous cases have been described in which different cell elements have been found in the large cells. There could no longer be any hesitation in interpreting these cases as instances of ingestion by leucocytes or similar cells.

The changes that the ingested elements undergo within amoeboid cells may be compared with those that take place in intracellular digestion. If the modifications of the particles ingested by the _Amoebae_ be studied side by side with those which take place in ingested cells in the process of resorption, a striking analogy may be observed. To establish this satisfactorily it is essential to begin with a study of intracellular digestion properly so called, especially as in this phenomenon we have the fundamental basis of the whole of the theory developed in this work.

In our first two chapters we have already cited examples of this intracellular digestion in the Protozoa (_Amoebae_, Infusoria, etc.) and in the plasmodium stage of the Myxomycetes. In all these cases it goes on in the organism, in a distinctly acid medium, by the aid of ferments which could be demonstrated in the _Amoebae_ and Myxomycetes, and which are analogous sometimes with trypsin, sometimes with pepsin.

In the lower Invertebrata we find the principal source of our knowledge of intracellular digestion in the digestive organs. This form of digestion is met with in Sponges, in the whole of the Coelenterates (Medusae, Siphonophora, Ctenophora, etc.), in the great majority of the Turbellaria (Planarians, Rhabdocoela), and in certain of the Mollusca (the lower Gasteropods). In the Invertebrata higher in the animal scale, intracellular digestion in the digestive organs becomes more and more rare, and sometimes it manifests itself only in the larval condition (_Phoronis_); ultimately it gives place permanently to digestion by juices secreted into the gastro-intestinal canal.

[Sidenote: [53]]

In his sketch of the comparative physiology of digestion, Krukenberg[76] sought to establish two types: protoplasmic or cellular digestion and secretory digestion. The former is effected, according to this observer, by a vital action independently of any production of soluble ferments. Secretory digestion alone, characteristic of the Vertebrates and of almost all the higher Invertebrates, is effected by means of these ferments (diastases or enzymes). Many observers, adopting this view, maintain that intracellular digestion presents a purely vital phenomenon essentially different from that of chemical digestion due to juices containing soluble ferments secreted in the gastro-intestinal canal. That this theory is absolutely erroneous the succeeding pages of this work will furnish ample proof.

[Sidenote: [54]]

The Protozoa, from their small size, are unsuitable for researches on the essential phenomena of intracellular digestion. Amongst animals higher in the scale the Planarians lend themselves most readily to the observation of this process. These flat worms are very common in both fresh and sea water and are easily fed in captivity. They are very voracious animals and, among other things, devour the blood of man or animals with avidity. One has merely to allow them to fast for a few days, and then to give them a drop of blood in order to see their digestive canal fill itself with this fluid (fig. 6). The white Planarian, _Dendrocoelum lacteum_, is well adapted for these researches. In a worm that has sucked blood from a Vertebrate, owing to its great transparency, the whole length of its intestine with its numerous ramifications may be seen. For some time this organ remains of a bright red colour, but gradually the tinge becomes brownish or faintly violet. These changes of colour recall those observed in effusions of blood in or under the human skin resulting from contusions. A microscopical examination of Planarians that have been fed with blood shows that the coloration of their digestive canal is due to red blood corpuscles in different stages of digestion. Immediately after the taking in of the blood by the Planarian all the red blood corpuscles are ingested by the epithelial cells of the intestine. Connected with the wall by slender stalks, these elements appear as large amoeboid cells whose free end projecting into the lumen of the intestine sends out protoplasmic processes which seize the red blood corpuscles and convey them into the interior of the cell. This goes on very rapidly, and in a very short time all the red corpuscles are found within the epithelial cells. As a result of the increase in volume of these cellular elements the intestinal cavity is completely occluded.

[Illustration:

FIG. 6. Young Planarian some time after having sucked goose’s blood. ]

[Sidenote: [55]]

[Illustration:

FIG. 7. Intestinal cell of a Planarian, filled with red blood corpuscles, undergoing digestion, of the goose. ]

Once inside the cells of the intestine the red blood corpuscles exhibit changes which are readily followed under the microscope. It is better still to feed the Planarians with the blood of those lower Vertebrates whose red corpuscles are nucleated. In my researches I have used the blood of the goose. The red blood corpuscles of this bird, when ingested by the epithelial cells of the intestine of Planarians, are usually collected into compact groups (fig. 7), only a few remaining isolated. The majority of these red corpuscles soon lose their normal appearance and contour; they become rounded and fused together, but the nucleus and the haemoglobin enable us to recognise them without any difficulty. Later the red colouring matter begins to diffuse into the digestive vacuoles which form around the corpuscles. These corpuscles empty themselves, retaining their nuclei and capsules, which shrivel more and more. The nucleus also undergoes almost complete digestion, its membranous layer alone persisting (fig. 8). Even several days after the digestion of the blood has begun one can still find _debris_ of perfectly recognisable red corpuscles, but the red colour has been replaced by a more or less pronounced brown tint. In the last stage of the digestive process, as the red corpuscles disappear, the protoplasm of the intestinal cells becomes filled with round vacuoles, containing brown irregular concretions—excreta—which are expelled into the intestinal cavity.

[Illustration:

FIG. 8. Digestion of red blood corpuscles of the goose within an intestinal cell of a Planarian. ]

This slow digestion of a substance usually so easily assimilable as blood takes place entirely within the epithelial cells of the intestine. Continuous microscopical observation demonstrates most clearly the complete absence of any extracellular digestion of the blood corpuscles in the intestinal content.

[Sidenote: [56]]

When goose’s blood mixed with blue litmus powder is given to Planarians, the coloured grains may be found some hours afterwards inside the epithelial cells of the intestine, but only a few of the blue litmus granules change colour, taking on a light violet tinge; the great majority retain their blue coloration. It might be concluded from this that in Planarians intracellular digestion is effected in a neutral or nearly neutral medium. If, however, the preparations of intestinal cells gorged with goose’s blood are treated with a 1% solution of neutral red, we at once notice that the red corpuscles and the vacuoles which contain them are stained bright red, assuming a tint similar to that given with picrocarmine staining (fig. 9). This colour reaction indicates, according to our researches on neutral red, an acid reaction, more feeble, however, than that met with in _Paramaecium_ and many other Protozoa.

[Illustration:

FIG. 9. Portion of an intestinal cell of a Planarian, treated with 1% neutral red. ]

Macerations of Planarians in normal saline solution to which has been added a small quantity of the red corpuscles of the goose’s blood exhibit _in vitro_ a very distinct solvent action on these corpuscles, which become rounded and lose their haemoglobin, this latter diffusing into the surrounding fluid, and at the close of the experiment there remain simply the membranes and the nuclei of the corpuscles.

[Sidenote: [57]]

The study of these Planarians shows us, then, that the food of these animals undergoes exclusively intracellular digestion in a feebly acid medium and by means of a soluble ferment, and it furnishes us with proof that typical intracellular digestion is essentially a chemical process due to the intervention of enzymes. Now there can be no question, here, of a protoplasmic action proper, but the branched digestive canal, so intimately associated with the parenchyma, cannot be completely isolated from the rest of the Planarian, and it is impossible to study _in vitro_ its digestive action apart from other tissues. To attain this end we must turn to animals of larger size and those in which the digestive organs can be isolated more easily. In the Coelenterata intracellular digestion is general. Many of them are so transparent that they can be examined _in vivo_. It is easy to observe that the particles of food are seized by amoeboid processes of the entodermic cells and that they pass into the substance of these elements there to be digested. For the systematic study of the digestive phenomena, however, it is not sufficient merely to examine all that takes place in the living animal. Experiment _in vitro_ is also necessary. For this purpose the Actinians or sea-anemones offer us really excellent material. As these animals are very common in all our seas and are easily kept alive for long periods in aquaria, they have been used for various researches, among others for the study of the process of digestion.

[Sidenote: [58]]

The Actinians are easily fed in captivity; they devour morsels of flesh, of shrimps, of mollusca and other marine animals with avidity. The ingenious English observers Couch and G. H. Lewes[77] long ago demonstrated that morsels of food when introduced enclosed in perforated quills or wrapped in test paper or gutta percha silk and swallowed by the anemones were afterwards ejected surrounded by mucus but with no trace of digestion. Having failed in their search for digestive juices in the large gastric or coelenteric cavity of the Actinians, Lewes concluded that digestion in these animals is effected in a purely mechanical fashion. The greatly developed muscles of the Actinians were supposed to squeeze the food and extract its fluid which is then absorbed by the walls of the general cavity. It was not until very much later that the problem of digestion in the Actinians could be resolved in any accurate and definitive fashion. More than twenty years ago I demonstrated[78] that the digestion in these polyps is intracellular. In order that a clear conception of this phenomenon may be obtained it may be useful to recall in a few words the fundamental features of the organisation of Actinians. They are cylindrical bodies, sometimes as large as the fist, attached by their base to stones, shells, or other submarine objects, and furnished at their free extremity with one or more series of tentacles. In the middle of this extremity is an elongated opening, the mouth, which leads into a spacious sac, often spoken of as the stomach. It is, however, only a kind of oesophagus, through which the food passes into the large coelenteric cavity which is divided by septa into numerous compartments lined by the entodermic epithelium. These septa give origin to many very long and tortuous filaments, spoken of as mesenterial filaments from their resemblance, a purely superficial one, to the mesentery of higher animals (fig. 10). When the Actinian is hungry it protrudes its tentacles in order to seize marine animals, which it conducts to its mouth. The lips and the oesophagus are used to estimate the quality of the capture, and if it is found unsuitable the anemone rejects it, first surrounding it with a layer of mucus. If however the food is found to be suitable, the Actinian retains it in its large cavity and throws around it a multitude of its mesenterial filaments. These penetrate it in all directions, and as their epithelial cells are capable of sending out amoeboid processes they seize and ingest the particles, which immediately enter the protoplasmic content. This work is done with such precision and nicety that the sea-anemone is able to extract the contents of a shrimp from the carapace, which latter alone it rejects.

[Illustration:

FIG. 10. Longitudinal section of an Actinian (after Hollard). ]

[Illustration:

FIG. 11. An Actinian in which carmine after absorption has passed into the mesenterial filaments. ]

[Sidenote: [59]]

The epithelium of the mesenterial filaments is therefore the organ of digestion in the Actinians. The nutritive parts of their prey pass into the amoeboid epithelial cells and there undergo a purely intracellular digestion. If we add to the shrimp-muscle or other food a little carmine or blue litmus powder, the mesenterial filaments ingest it also and become pigmented. After eating carmine they assume a very brilliant rose colour (fig. 11); blue litmus colours them rose violet. This change of colour in the interior of the cells of the filaments indicates a decidedly acid reaction of their contents[79]. When one adds to the mesenterial filaments which are carrying on the process of digestion a drop of a 1% solution of neutral red they assume various shades of red (fig. 12).

[Illustration:

FIG. 12. Portion of mesenterial filament of an Actinian, stained with 1% neutral red. ]

This intracellular digestion in the Actinians has been confirmed by several observers, amongst whom may be cited Chapeaux[80] and Bjelooussoff[81]. It has often been asserted, however, that, along with a digestion in the interior of the cells of the mesenterial filaments, there is, in the Actinians, a secretion in the coelenteric cavity of their body of fluids which digest nutritive matter by means of a soluble ferment. A ferment similar to trypsin has been extracted from Actinians by Léon Frédéricq and Krukenberg. But, in presence of contradictory assertions, it remained undecided whether, in the enzymatic digestion, this ferment does its work in the fluid of the coelenteric cavity or whether it represents the active factor in intracellular digestion.

With the object of definitely elucidating a problem of such general importance, Mesnil, the superintendent of my laboratory, has been good enough to carry out a fresh series of experiments on the digestion of the Actinians and has studied this process not only in animals kept in captivity in aquaria but also in Actinians living under natural conditions in the sea[82].

[Sidenote: [60]]

As intracellular digestion is of interest to us specially in connection with the resorption of formed elements in the tissues and cavities of animals, Mesnil directed his attention to the digestion of the red corpuscles of the blood. He made use of the red corpuscles of several species of Vertebrata, but he made a special study of the digestion of nucleated red blood corpuscles. These corpuscles are very delicate, and may even undergo a certain degree of maceration in ordinary sea water. In spite of this these red corpuscles are not digested in the coelenteric cavity of the Actinians but, once ingested by the entodermic cells of the mesenterial filaments, they are completely dissolved by the intracellular digestion. Mesnil also observed that fibrin is not digested except in the cells of the filaments. The facts cited by Chapeaux in favour of an extracellular digestion in the fluid of the coelenteric cavity in no way support his hypothesis, and reduce themselves, according to Mesnil, to a digestion by the diastase of blood itself fixed by the fibrin, after the bleeding, at the moment of the formation of the clot.

For a certain period the red corpuscles may be met with inside the cells of the mesenterial filaments. They are ingested in their normal state—oval red corpuscles with a nucleus. As several hours are required for the ingestion, it is evident that the fluid of the coelenteric cavity has been incapable of attacking the red corpuscles. In the protoplasm of the entodermic cells the red corpuscles become rounded, their walls become permeable, and the haemoglobin begins to diffuse from them. It passes first into the vacuoles of the digestive cells and is then, in part, ejected into the general body cavity. The haemoglobin is transformed into a green substance which reminds one of biliary pigment. The membranes and nuclei of the red corpuscles are also digested and ultimately disappear completely.

The digestive cells of the entoderm ingest not only blood corpuscles or fibrin, but also fragments of muscular fibre and particles of carmine and litmus. These latter, as already stated, indicate a marked acid reaction.

[Sidenote: [61]]

In the Actinians, then, the mesenterial filaments, or rather their entodermic portion, represent the real organ of intracellular digestion. There are indeed other regions of the entoderm which also carry on this function, but in an insignificant degree as compared with the mesenterial filaments which are capable, however, not only of ingesting and digesting solid substances, but also of absorbing solutions. Mesnil has demonstrated this by injecting soluble colouring matters, such as eosin, carminate of ammonia, etc., into Actinians. These solutions, although in great part absorbed by the digestive cells of the mesenterial filaments, can, however, also be retained by other elements, amongst others, the cells of the ectoderm.

As the digestion of the food-particles goes on within the entodermic cells of the mesenterial filaments and as these organs can easily be isolated from the rest of the Actinian, Mesnil was able to study with great precision and care the phenomena of digestion outside the organism. With this object he prepared extracts of the filaments in sea water and studied their action on various nutritive substances. He confirmed the discovery of a soluble ferment made by Léon Frédéricq and demonstrated that it is capable of digesting albuminoid substances (fibrin, coagulated albumen) in media which are neutral, slightly alkaline or weakly acid. In this respect the _actino-diastase_ (the name given by Mesnil to the soluble ferment of the Actinians) approaches most nearly to papain. On the other hand, it is distinguished by its greater sensitiveness to an excess of acid and also by its more powerful action on coagulated albumen.

The actino-diastase acts vigorously at any temperature between 15° and 20° C., but the optimum temperature for its digestive action is between 36° and 45° C. Higher temperatures weaken the diastatic power, and heating to 55–60° C. inhibits it completely. Among the products of the digestion of albuminoids by actino-diastase, Mesnil, like his predecessors, found not only a notable quantity of peptone but also products of the disintegration of the albuminoid molecule, such as tyrosin and proteino-chromogen. Consequently actino-diastase resembles Mouton’s amoebo-diastase in certain respects.

[Sidenote: [62]]

The nucleated red blood corpuscles of the lower Vertebrata are very convenient objects on which to observe the process of intracellular digestion within the cells of the mesenterial filaments. Mesnil has also studied them _in vitro_ under the influence of actino-diastase. Under these conditions the phenomena of digestion recall very clearly those that have been observed within the digestive cells. The oval red corpuscles of the fowl and goose become spherical as a result of the solvent action on their membrane, and the haemoglobin diffuses into the fluid. The membranes and the nuclei of the corpuscles are, however, little altered and may be recognised under the microscope. The difference between this and digestion within the cells reduces itself to a more feeble digestive action of the aqueous extract. It is evident that the preparation of this extract is only capable of bringing into prominence a certain proportion of the actino-diastase contained in the entodermic cells of the filaments.

Mesnil has fed the same Actinians with repeated doses of blood with a view to make out whether the cells, under these conditions, acquire any special aptitude for the production of the actino-diastase. Notwithstanding numerous attempts, he could never assure himself that this takes place; the rapidity with which the red corpuscles were dissolved by the extract of the mesenterial filaments was the same whether this was prepared from Actinians that had been several times fed on blood or from those that had received none at all.

From what I have just described no doubt can exist that intracellular digestion is not a “protoplasmic” process essentially different from that which is brought about by the digestive juices secreted in the intestinal canal. In both cases we have a diastatic action, due to soluble ferments, produced by living elements. In intracellular digestion, however, the diastases carry on digestion in the interior of the cells, principally in the vacuoles, whilst in extracellular digestion this process goes on outside the cells, in the lumen of the gastro-intestinal canal.

It cannot be doubted that, in the animal scale, intracellular digestion represents an earlier and primitive condition for the solution of the food substances. This follows from the fact that it is widely distributed amongst the lowest animals, such as the Protozoa, Sponges, Coelenterata and Turbellaria. Intracellular digestion only gives way step by step to digestion by secreted juices. The higher Invertebrata furnish us with conclusive testimony on this point. Thus, among the gasteropod Mollusca, there are some which exhibit the two modes of digestion in the same animal. In _Phyllirhoë_, a beautiful mollusk, without a shell and quite transparent, which floats on the surface of the sea, the food can be seen passing into the cavity of the digestive canal, where it undergoes a preliminary digestion by secreted juices; the result is a magma of small solid particles which are at once seized by the amoeboid epithelium of the coecal appendages, two on each side of the body. Intracellular digestion then completes the process and ends by dissolving the nutritive substances and reducing them to their final stage previous to absorption. On adding to the food some particles of carmine these may be found along with the digestible particles in the interior of the epithelial cells of the coeca.

[Sidenote: [63]]

This example furnishes us with a real link between primitive intracellular digestion and the perfected and derivative extracellular digestion. In the same group of Gasteropods may be followed out several stages of this evolution so that in the higher representatives of the group, such as the slugs and the snails, we meet with digestion carried on only by secreted juices in the gastro-intestinal contents. In these Mollusca a voluminous glandular organ, the liver, which is certainly derived from coecal appendices similar to those of _Phyllirhoë_, is now met with. Regarded from this point of view the liver is, as Claude Bernard has stated, an organ of second digestion. I think that a detailed study of the liver of the Mollusca, guided by this idea, will give results of considerable importance.

In the Vertebrata intracellular digestion in the gastro-intestinal canal almost disappears and is replaced by digestion carried on by means of ferments contained in secreted juices. We cannot, of course, offer to the reader anything like a complete account of this extracellular digestion in the higher animals. It is necessary, however, to draw attention to several aspects of this function which have been established, thanks to the progress made during recent years, in obtaining digestive juices and in the study of their action.

[Sidenote: [64]]

For the study of intracellular digestion the sea-anemone is the most suitable animal for our purpose; for that of extracellular digestion the dog. In this latter animal, an omnivorous flesh-eater, the food substances are treated by digestive juices of great activity which contain a whole series of soluble ferments. The stomach secretes two of these: rennet and pepsin. The pancreas elaborates three: trypsin, amylase and saponase, which act on the three main groups of food substances. To these the small intestine adds a special ferment, described by Pawloff[83] under the name of enterokynase. Every one recognises the proteolytic function of pepsin and trypsin and the analogies and differences between these two diastases. Nor need I dwell on amylase or on the ferment which saponifies fats. But enterokynase merits special attention in connection with the study of immunity. Pawloff entrusted to his pupil Chépowalnikoff the study of the digestive _rôle_ of the intestinal juice concerning which, up to this, very little was known. It was known indeed that this juice contained weak saccharifying and inverting ferments, but it was generally regarded as a secretion of little importance. Chépowalnikoff[84] has demonstrated that this view is absolutely erroneous. The intestinal juice fulfils the very important function of accelerating the action of the three pancreatic ferments. The duodenal juice of the dog, especially, contains enterokynase. When this juice is mixed with a pancreatic juice that by itself actively digests fibrin and albumen, digestion takes place still more rapidly, the action being from three to four times as great. The part played by the intestinal juice becomes even more evident when it is mixed with a pancreatic juice that has little or almost no activity, as is the case of that from dogs that have recently been operated upon. Thus pancreatic juice, which has no action upon albumen, digests it promptly when a certain quantity of duodenal juice is added. When Chépowalnikoff took 500 c.c. of inactive pancreatic juice diluted with 500 c.c. of water or soda solution and added to it but a single drop of intestinal juice, the mixture exerted a manifest digestive action on coagulated albumen.

If, in place of pancreatic juice, we take the aqueous or glycerinated extract of the pancreas, which by itself exerts a very insignificant digestive action on albumen, and add to it intestinal juice, digestion takes place immediately. If it be admitted, as several physiologists maintain, that the inactivity of the pancreas is due to the fact that we have zymogen present in place of trypsin, one might conclude with Chépowalnikoff that “the intestinal juice possesses the power of transforming the zymogen into trypsin, and that this transformation takes place in a much more marked degree than in the presence of acids or the oxygen of the air” (p. 137).

The intestinal juice, from whatever region of the small intestine it be derived, exercises an undoubtedly favourable influence on the digestion of starch by the pancreatic juice, but this action is much more feeble than that on trypsin digestion. The action of the intestinal juice on the saponification of fats is even less marked. But here it is to the bile that the more important _rôle_ is transferred. This fluid also augments the activity of the pancreatic juice, but in a manner different from the intestinal juice, for it acts especially by accelerating the digestion of fatty substances.

[Sidenote: [65]]

The action on the pancreatic digestion is not in any way interfered with when the bile is heated to boiling point. On the other hand the intestinal juice, under these conditions, completely loses its accelerating _rôle_. It follows from this, as has been formulated by Pawloff, that, in the intestinal juice, the existence of a soluble ferment which is destroyed by heat must be admitted; to this ferment he proposes to give the name of enterokynase. Without exercising a digestive power on any of the alimentary substances, it may act as a ferment of the pancreatic ferments.

Delezenne, at the Pasteur Institute, has repeated Chépowalnikoff’s experiments. He has confirmed the accuracy of his results and has added new data of great importance, not only as regards the physiology of digestion but also in relation to the study of immunity. Enterokynase appears from Delezenne’s experiments to be a true ferment; carried down by the same precipitants (collodion, phosphate of lime, alcohol) which enable us to obtain the greater number of the known ferments; it is sensitive to high temperatures, and even that of 65° C. is sufficient to do away with the greater part of its activity. Yet another property of enterokynase, which it possesses in common with the soluble ferments and which has for us a very special interest, is the facility with which it attaches itself to fibrin. By means of flakes of this substance we can at any time remove from a fluid the whole of the enterokynase contained therein. This fixative property is very important in connection with the part which enterokynase plays in digestion. The fibrin to which it has become attached absorbs trypsin with great avidity. If we introduce flakes of fibrin impregnated with enterokynase along with other flakes which have not been in contact with this ferment into a solution of trypsin, the former are digested with great rapidity, whilst the latter do not undergo any change. The fibrin that has fixed enterokynase is capable of clearing a fluid of its trypsin. On the other hand, that which has not been acted upon by the intestinal juice leaves it there almost unaltered.

[Sidenote: [66]]

It is of the utmost importance that we should inform ourselves as to the origin of the enterokynase of the intestinal fluid. This fluid, when obtained from a fistulous opening, for example, contains mucus and a considerable amount of _débris_ of various kinds of cells. What are the elements which furnish such a remarkable ferment? Delezenne has obtained a very precise answer to this question. The enterokynase is not contained in the mucus and is not secreted by the intestinal glands; it comes from the lymphoid organs.

If the small intestine of a fasting dog be washed carefully with water all the pre-existing enterokynase is removed from it. The Peyer’s patches are then removed and treated with chloroform water. The other parts of the small intestine are similarly treated. This fluid dissolves the enterokynase, as it does the other soluble ferments. We find that the Peyer’s patches furnish enterokynase, but that the rest of the intestine, including Lieberkühn’s glands, give none.

We know that the Peyer’s patches are lymphoid organs in which are a large number of amoeboid mononucleated cells, and that these elements are even capable of ingesting foreign bodies and of submitting them to intracellular digestion. It is therefore not at all astonishing that Delezenne should have succeeded in finding enterokynase in the mesenteric glands of several Mammals (dog, pig, rabbit). These glands, when treated by the method just mentioned, yield a substance which assists the action of trypsin just as does the intestinal juice. Having reached this point, Delezenne asked himself whether the mononucleated white corpuscles, so closely allied to the mononucleated cells of the lymphoid organs, may not also contain enterokynase. With the object of settling this point he collected exudates that were rich in mononucleated leucocytes; in these also he found this same soluble ferment. Moreover, the leucocytic layer of the blood showed itself equally capable of increasing, very energetically, the action of trypsin.

The results of the old experiments carried out by Schiff and by Herzen on the adjuvant _rôle_ of the extract of the spleen in pancreatic digestion, must without doubt be ranged alongside those we have just indicated. In fact the mononucleated cells of the spleen, like those of Peyer’s patches and of the mesenteric glands, contain a substance which acts like enterokynase. Delezenne has given us a definite demonstration of its presence and action.

[Sidenote: [67]]

In intracellular digestion it is the chemical side which has been most difficult of demonstration. The purely physiological functioning, the sensitiveness of the digestive cells and the amoeboid movements of their protoplasmic processes are, on the other hand, so manifest that it has even been suggested that intracellular digestion should be looked upon as a protoplasmic phenomenon purely vital in character.

In extracellular digestion through the agency of secreted juices we have a very different condition. Here the chemical side is the striking feature, the physiological factor being veiled more or less completely. Nevertheless, thanks to recent advances and above all to the labours of Pawloff’s disciples in St Petersburg, this problem has been elucidated in a very remarkable fashion.

The secretion of digestive fluids follows definite laws, the most potent factor being the reflex action of the nervous system. To use the expression of Pawloff, the study of the process of salivary secretion has revealed a real psychology of these organs. You may fill the mouth of a dog with small polished pebbles or with snow; you may pour into it very cold water—the saliva will not flow. But merely allow the animal to see sand in the distance—the glands at once begin to secrete fluid saliva. Tempt the dog with flesh—and immediately a thick saliva appears; show him dry bread—saliva is secreted in abundance, even if the dog has no great desire to eat.

The same phenomena may be observed in the stomach. Mechanical stimulation by inert bodies, such as stones, provokes no secretion; but the suggestion of a meal or the sight of food is sufficient to call forth a large quantity of gastric juice. The quantity and quality of the gastric juice are regulated by the quantity and quality of the food. Bread given to a dog provokes the secretion of a gastric juice endowed with the greatest digestive power. That which flows after the ingestion of milk contains only one-fourth as much pepsin.

[Sidenote: [68]]

In spite of these differences in the gastric secretion in relation to food, Pawloff and his pupils have never been able to assure themselves that there was any prolonged and chronic adaptation of the gastric function. They were struck by the uniformity of the digestive power of a great number of their dogs. Samoïloff[85] had under observation three dogs placed on different diets. In spite of the very long periods during which these diets were given, the gastric juice, in all the dogs, presented the same properties and manifested no appreciable difference. This result harmonises with that indicated above as obtained in the Actinians fed with blood by Mesnil. In spite of repeated feedings on blood from the same species of animal, the extract from the mesenterial filaments was in no way different from that of the fasting Actinians used for control.

The pancreatic secretion is, in many respects, a more perfect type. We have here to do with the principal agent in the digestive function, without which the organism could not continue to exist. The advances made in surgery have enabled us to remove the stomach, first in the dog and then in man, and there are already several persons[86] from whom the stomach has been removed and who, in spite of this operation, have continued to live. A portion of the small intestine may also be removed, but, in order that life may not be endangered, a considerable portion of it must be left intact. It is evident then that the pancreatic digestion is an admirably organised function both in animals and in man. One of the main regulators of this process of digestion consists in the great sensitiveness of the intestinal mucous membrane. Just as the organs of the buccal cavity possess in the specific sense of taste an excellent means of discrimination in the choice of foods, so the mucous membrane of the small intestine is endowed with a special sensitiveness, comparable to the chemiotaxis of unicellular organisms and of the cells of more highly developed organisms. Hirsch and Mehring have satisfied themselves that the passage of the contents of the stomach through the pyloric orifice depends on a reflex mechanism which proceeds from the upper reaches of the small intestine. To the researches of the school of Pawloff, however, we owe what light has been thrown on this question. The duodenal mucous membrane is endowed with a well-developed chemiotaxis for acid substances. The passage of the acid content of the stomach into the duodenum determines this chemiotaxis and brings about a secretion of alkaline juice which neutralises the acid. This contest between acid and alkali forcibly calls to our mind the analogous phenomena in those plants that defend themselves against the alkaline secretions of parasites by the production of an acid (see Chapter II). As in these lower organisms, this battle of the chemical secretions is regulated by the action of living and sensitive parts.

When the acidity of the mass which passes through the pylorus is too marked, the reflex contraction starting from the duodenal mucosa arrests its passage. Then takes place a neutralisation of the acid, thanks to the alkaline secretion, and the pylorus is again allowed to open. This mechanism thus regulates the passage of the contents of the stomach into the duodenum, the passage taking place in instalments.

[Sidenote: [69]]

The sensitive intestinal mucous membrane can estimate not only the degree of acidity, but also the other chemical characters of the aliments which pass into the duodenum. This chemiotaxis is, as it were, the starting-point of the reflex action which excites the pancreatic secretion with its contained three ferments. The passage of bread through the pylorus excites the secretion of a juice very rich in amylase and very poor in saponase. The passage of milk into the duodenum brings forth, on the other hand, a juice very much richer in saponase but poorer in amylase and in trypsin. Flesh-meat provokes the secretion of a pancreatic juice which is less rich in amylase than the juice poured on bread, but richer in saponase. Fat causes the secretion of a juice still richer in saponase than is the juice poured out in the presence of bread or milk. These facts now carefully established—especially by Walter[87]—demonstrate that the pancreatic function is carefully regulated as regards its adaptation to the characters of the food substances on which it is to act. Such adaptation may even become permanent.

Whilst, as already stated, the stomach, under the influence of a fixed diet, is incapable of effecting any lasting modification in the composition of its secreted juice, the pancreas may reach this degree of perfection. When a dog is fed for several weeks on bread or on milk and is then placed on flesh diet its pancreatic juice is found to become progressively richer in trypsin. Whilst this augmentation of the proteolytic power is being brought about, the juice becomes poorer and poorer in amylase. Wassilieff[88] has carried out a large number of experiments on this point and has demonstrated a very remarkable adaptation of the pancreatic juice to the wants of nutrition, an adaptation that may become permanent. A dog which has been accustomed to digest bread and milk adapts itself to this nourishment: its pancreatic juice contains less and less trypsin, but, on the other hand, becomes richer in amylase. Pawloff observed that in dogs great variations in the composition of the pancreatic juice are often present; this he attributes to the diet to which these animals had been previously subjected.

[Sidenote: [70]]

Not only does the quality of the digestive juices accommodate itself to the wants of digestion; their quantity also undergoes variations according to the part that these juices have to play. Thus, Pawloff has observed that his dogs secreted a saliva which was very fluid and very abundant when he gave them acids, bitter substances or other substances they did not like. On the other hand, the presence of food in the mouth, or even the sight of it, excited the secretion of a thick saliva containing a large quantity of mucin. In the first case the part played by the saliva was that of diluting the injurious substances as much as possible, in the second that of facilitating the deglutition of the food.

In general the organism manifests a tendency to produce more digestive ferments than it actually needs for digestion. It is for this reason probably that they are often found outside the digestive canal. Among these ferments pepsin and amylase, especially, have been definitely proved to be present in the urine of man and of some mammals, notably the dog. The data as to rennet and trypsin are not so well established. But, as several of these ferments, such as amylase and trypsin, may be derived from several sources in the organism, their elimination by the urine is less important for the thesis I have just formulated than is that of pepsin.

Pepsin was found in the urine by Brücke exactly forty years ago. It is more frequently found in the morning urine, but is absent from that passed immediately after the principal meal. Leo and Senator[89] found only traces of pepsin during the prolonged fast of the Italian Cetti; but the day he broke his fast they were able to demonstrate the presence of a considerable quantity of this ferment in his urine.

[Sidenote: [71]]

Delezenne and Froin, with the object of seeking the source of the urinary pepsin, extirpated the stomach of a dog. After the animal had recovered, they fed it well and examined its urine at different periods of the day. By the methods which had shown the presence of pepsin in all the normal dogs taken as controls they could never discover the faintest trace of this diastase in the urine of the dog that had been operated upon. On the other hand, the urine of a dog whose stomach had simply been isolated, contained very much the same quantity of pepsin as that of normal dogs. This experiment proved among other things that the pepsin, before it could be eliminated by the kidneys, must have been re-absorbed by the wall of the stomach. From these data, combined, it must therefore be admitted that the pepsin found in the blood and which passes thence into the urine can only be of gastric origin. As it serves no useful purpose in the organism we must conclude that a portion of the pepsin, secreted by the stomach and not used for digestion, has been rejected as superfluous.

The study of the digestive function of animals gives us information on a large number of points of the highest importance for the comprehension of immunity. Intracellular digestion, a function so widely distributed in the lower animals, is very intimately connected with the phenomena which are observed when micro-organisms are destroyed in the animal organism. Extracellular digestion furnishes us with information concerning many of the features of progressive adaptation, similar to those which are observed in connection with acquired immunity.

When we examine the phenomena of intracellular digestion and those of secretory digestion as a whole, we see that, in both, the chemical processes are subjected to the influence of the living parts of the organism. In the lower animals, it is the protoplasm of the amoeboid cells which regulates the chemical processes in digestion; in the higher animals, this _rôle_ is taken by a very complicated apparatus, in which the nervous system plays a predominant part.