Part 13

But it is well to remember that man’s structure is riddled with evidences that he passed from an ancestral, quadrupedal condition, through the semi-erect to his present upright posture, slowly and laboriously. His erect attitude, geologically speaking, is a very recent accomplishment, and his anatomy, therefore, reveals many imperfect adaptations to his newly acquired posture. These imperfect adaptations are the sources of many grave diseases in mankind. It would require too technical a knowledge of anatomy to explain these imperfect adaptations, and I will therefore simply mention _rupture_ and _uterine displacements_ as due to imperfect adaptations to the upright attitude.

The common origin of man and the ape accounts for many interesting and otherwise inexplicable facts in anatomy. There is, for instance, a muscle that is normally present in the orang-outang known as the Opponens Hallucis. This muscle enables the orang to oppose his big toe to the other toes, just as we can oppose our thumb to the other fingers of our hand. This muscle is absent from the foot of man ordinarily. But occasionally it is found in man, in the dissecting-room, as a rarity—as an anomaly. The question naturally arises, why should this muscle be present normally in the orang and absent normally in man, occurring in the latter only as an abnormality? The theory of evolution gives the only rational answer. The man-like, ape-like generalized ancestors of man and the orang possessed this muscle, which was useful to them in grasping the branches of the trees among which they lived. These ancestors used the feet and hands alike for purposes of grasping (prehension) and locomotion. But those descendants that evolved more and more man-ward used the feet more for purposes of locomotion and less for grasping, while they used the hands more for grasping and less for locomotion, until, finally, man was created—a creature that uses his feet exclusively for locomotion, and the hands entirely for grasping. Through disuse, therefore, the _opponens hallucis_ gradually disappeared in man; so that now it occurs only as a rare abnormality. The hereditary units that make this muscle still lie dormant in most men are usually so weak, through disuse, that they do not develop. Some unusual stimulus occasionally causes the latent hereditary units to develop and makes it appear in man. The same is the case with many other muscles and structures that are normal in the modern anthropoid apes, and only occur as rarities in man. The appearance of those muscles in man are instances of _atavism_, i. e., reversions to conditions that were normal in the ancestors of man and the apes, as they are still normal in the latter.

USELESS SCAFFOLDING LEFT IN THE BODY.

Man, in his post-natal growth, as well as during his embryological development, exhibits reminiscences of his animal ancestry. In the structure and movement of the new-born babe, as well as in the adult frame, we find continuous witnesses to the ancient animal strain.

On the theory that men in bygone ages were closely allied to simian creatures in habit as well as structure; that they led an arboreal life; and that, like the baby-monkeys to-day, the baby-men of other ages clung to their mothers as they climbed among the trees, Dr. Louis Robinson predicted that a =baby’s power for grasping= would likely be found to equal that of a young monkey which had reached a corresponding period of growth. He tested a large number of new-born infants in reference to this power by extending his finger or a cane, to imitate the branch of a tree, and observed how long they would hang there without any other support (Plate XI). He made experiments on about sixty children under a month old. About thirty of the children experimented upon were not over an hour old. Dr. Robinson states that each of the infants, with two exceptions, was able to hang to the finger or cane by its hands, like an acrobat from a horizontal bar, and sustain the whole weight of its body for at least ten seconds. Twelve of the infants, less than an hour old, held on for half a minute before the grasp relaxed; while four of this age held on for one minute. Over fifty of the infants when four days old could continue the grip for half a minute. Three weeks after birth the faculty for holding on reached its maximum, for at this age several succeeded in hanging on for a minute and a half; two held on for over two minutes; and one infant held on over two minutes and a half. One infant that was less than an hour old hung by both hands to Dr. Robinson’s finger for ten seconds, and then deliberately let go with his right hand, as if to seek a better hold, and continued his grasp with the left hand only, for five seconds longer. In none of these experiments did the limbs of the infants hang down in the attitude of the erect position, but the thighs were invariably in the baby-monkey attitude, at right angles to the body. The doctor says that this attitude and the disproportionately large development of the arms compared with the legs give the photographs of the infants a striking resemblance to a well-known picture of the celebrated chimpanzee, Sally, at the Zoölogical Garden in London. In these experiments the infants very seldom gave any sign of distress, and uttered no cry until the grasp began to give way. The fact that the flexor muscles of the forearm of a new-born infant show such remarkable strength while the other parts of the muscular system are so conspicuously weak and flaccid,—that they are able to perform a feat of muscular strength that will tax the powers of many a healthy adult,—can be explained only on the theory of inherited instinct from simian ancestors that lived in trees. This instinct is no longer useful to an infant. It is a vestigial instinct, a useless scaffolding in its life history.

[Illustration: PLATE XI.—Illustrating the grasping power of infants. Two infants, ten and thirteen days old, respectively, supporting their weight by the hands only (vestigial instinct.) Reproduced from a photograph taken by Dr. Louis Robinson. By courtesy of the Open Court Publishing Company.]

=Club-foot.= There is an ordinary case of malformation in the foot of a child known as club-foot. The most common kind of this deformity is that where the sole is turned inwards and upwards and the heel is raised. Before birth all children pass through this condition as a perfectly normal and natural one, and only gradually outgrow it (evolve beyond it). But some children fail to evolve beyond this condition and have club-feet throughout life, unless relieved by the surgeon. It is a very instructive fact that this particular form of club-foot is the normal condition of the adult gorilla and orang-outang. The foot of every child passes through this gorilla phase, and if it does not develop beyond this phase it retains the simian characters, and we call it an abnormality. In this abnormality the anatomist finds that those bones that enter into the formation of the ankle joint have the pronounced anatomical characters of the adult orang-outang.

=Ribs.= Adult man possesses twelve pairs of ribs. The chimpanzee and gorilla possess fourteen pairs. An older comparative anatomy predicted that in an early embryonic condition man would be found to possess thirteen or fourteen pairs. The prophecy has been verified.

=Hair.= The apes have hair over the entire body. At the sixth month of the embryonic development the human fœtus is thickly covered with a somewhat long, dark hair over all the body, except those parts that are uncovered in the apes, viz.: the palms of the hands and the soles of the feet. This covering of hair is called lanugo. Since it covers all the body except the points noted, it extends, of course, all over the ears, face and forehead. It is usually shed before birth. It is a simian characteristic, and sometimes fails to disappear, but persists and develops greatly. Therefore there are occasionally found such men (“dog-faced men”) as the Russian Jeftichjeff. The Ainos of one of the Japanese Islands also possess this extreme hairiness.

=Vermiform Appendix.= There are a number of vestigial structures in man that are not only useless but even a menace to life. The most striking of the vestigial structures that come under this category is a portion of man’s large intestine which is called the Appendix Vermiformis. This useless structure is a veritable death trap. In some animals, such as the herbivorous ones, the appendix is very large, sometimes longer than the body itself, and is of great use in digestion. But in man it has shrunken to a small rudiment varying from two to six inches in length, which is very liable to a grave form of disease that frequently causes death unless timely treated by the surgeon. In the early embryo the appendix is equal in caliber to the rest of the bowel, but at a certain date ceases to grow _pari passu_ with it. At birth it has become a small rudiment of the large intestine. In the new-born infant the appendix is often of the same size as it is in the adult. This precocity of an organ is always an indication that it was of great importance to the ancestors of the human species.

=Tail.= Man, like the anthropoid apes, has no external tail; but, exactly like them, he has a rudimentary one concealed beneath the skin. The embryos of man and the ape at an early stage of growth possess a very conspicuous tail, which is even longer than the limbs. In the embryo of man even the muscles for wagging the tail are still found. In the adult man these muscles are represented, normally, by bands of fibrous tissue. In the dissecting-room one occasionally finds these muscles well developed in the adult man. Man and the anthropoid apes have descended from more primitive simian ancestors that possessed tails.

=Hearing.= Prominent among vestigial structures, though less easy for beginners to understand, are those that point to piscine ancestors and which, therefore, smack of the sea. Embryology points indubitably to the fact that the ancient, geologic progenitors of man once lived a marine life. In the history of the globe there was a time when all the animals lived in the sea. Land animals appeared as later creations. Man, in evolving from the primitive protozoan, passed through a marine-worm phase and finally, through the ages, attained to the fish stage. The chief characteristic of a fish is its apparatus for breathing the air dissolved in the water. This apparatus consists of gills—strong bars with delicate, highly vascular, fringe-like curtains hung on them, and through which the blood is continually circulating. The circulating blood throws out its impure gases and takes in from the water the pure air, thus breathing. These bars or arches are five or seven in number in many fishes. Slits extend from the surface of the fish between the bars to the throat, so that the water which the fish takes into its mouth is forced out between the bars, thus bathing the delicate curtains on them by which air is breathed from the water. Sometimes the slits between the bars are open and unprotected, as in the sharks; but in the modern fishes (teleosts) they are protected by a lid (operculum). If these slits did not exist in the neck all fishes would quickly perish. They are of so great use to the fish that Natural Selection has taken exceptional care in perfecting their mechanism.

It is one of the most interesting facts in evolution that these slits in the fish’s neck are still represented in the neck of man. One of the most prominent features in every mammalian embryo is the presence of four clefts of the old gill-slits. So persistent are these characters that children are occasionally born with persistent fissures leading to the throat, so that milk, when swallowed, will come out on the neck through an opening. Thus we have a persistent piscine characteristic as an abnormality in the child.

When the fish-like ancestors of man left the water the elaborate breathing apparatus was no longer needed for respiration. Nature, in creating new adaptations for the land animal, did not discard the elaborate gill apparatus that had been evolved through the ages; but utilized this old apparatus for the new adaptations. Nature is exceedingly economical and does not discard old organs when they can be molded for new functions.

In the course of ages, through minute gradations, the first gill-slit and portions of its adjacent bars were molded for purposes of hearing. In man there are two passages leading to the drum or middle ear; one is the external auditory canal (the opening which is seen in what is popularly called the ear), and the other is a canal leading from the throat to the middle ear. In the adult these two channels are partitioned off from each other by the membrane of the drum. These canals are the counterpart or homologues of the spiracle associated in the shark with the first gill-slit. The external ear is developed by the coalescence of six rounded tubercles appearing in the bars or branchial arches that surround the first gill-slit. In the course of ages the remaining gill-bars (branchial arches) were also modified for special uses.

In relation with the external ear of man are found rudimentary muscles that are no longer useful and ordinarily are not under the control of the will. These muscles are the exact counterparts of well developed functional muscles found in great numbers of the lower animals. They are present in man as vestigial structures, because he is descended from animals in whom these muscles were well developed and functional.

The anatomy of man reveals so many additional vestigial structures that we may look upon him as a museum of obsolete anatomies; he is an old curiosity shop containing many discarded tools, many outgrown and aborted organs. The lower animals as well as man contain so many useless (vestigial) structures among their useful organs, and they are so significant of a former state of things in which they were useful, that anatomists are willing to stake the theory of evolution upon their presence alone. Evolution explains a multitude of other facts about man that are inexplicable on any other theory.

[Illustration: FIG. 19. Brain of Fish (Bluefish). A, dorsal view; B, side view; of, olfactory lobe; cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla; th, thalami.]

In addition to pointing out the possible track along which man has evolved from a primitive protozoan it would be interesting as well as exceedingly instructive to trace the development of each structure and organ in his body. But the subject is a vast one, and cannot be presented here even in briefest outline. Yet it will be very valuable to trace the unfolding of one organ, and that the highest, as a sample of what occurs with every part of the body. I refer to the _development of the brain_.

THE DEVELOPMENT OF THE BRAIN IN PHYLOGENY AND ONTOGENY.

Fig. 19 represents the brain of an average bony fish. It consists of six swellings in a line, one before the other. Beginning from the end towards the spinal cord, they are designated as follows, viz.: a single median lobe, the medulla (Metencephalon), m; then in front of this is another single median lobe, the cerebellum (Epencephalon), cb; then the optic lobes (Mesencephalon), right and left, ol; then the thalami (Thalamencephalon), which are small and hidden from view by the encroachment of the two adjacent segments; then the cerebrum (Prosencephalon), cr; then, finally, the olfactory lobes (Rhinencephalon), of. In this fish the largest of the segments are the optic lobes, ol.

[Illustration: FIG. 20.—Brain of Reptile (Turtle). A, dorsal view; B, side view; of, olfactory lobe; cr, cerebrum; th, thalami; ol, optic lobes; cb, cerebellum; m, medulla.]

The reptile’s brain (Fig. 20) shows similar parts with the same serial arrangement. The reptile is a higher creature, a more intelligent animal, than the fish; and in consonance with this fact the cerebrum, cr, is the larger and more dominant part of the brain instead of the optic lobes, ol.

[Illustration: FIG. 21.—Brain of Marsupial (Opossum). A, side view; B, dorsal view; of, olfactory lobes; cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla. Thalami concealed from view by the backwardly extended cerebrum; also the optic lobes are partially covered by the cerebrum.]

[Illustration: FIG. 22.—Brain of Lemur (Lemur nigrifrons). A, dorsal view; B, side view; cr, cerebrum; cb, cerebellum; m, medulla.]

In the marsupial (Fig. 21), a more intelligent animal still, the cerebrum, cr, has grown so large that it extends backwards and partially covers the optic lobes. It is to be observed that in the marsupial the cerebellum, cb (like the cerebrum, cr), has evolved to a higher phase. It consists of a median lobe, cb, which is larger than the median cerebellum of the lower creatures mentioned, and of two lateral lobes, one on either side, which have been acquired in the course of evolution. The median lobe, the homologue of the single, median cerebellum of lower animals, is larger than the lateral ones. The cerebellum of the marsupial has its surface increased by fissures, while that of the fish and reptile is smooth. The fissured cerebellum is a higher evolution than the smooth ones. In the groups of animals referred to so far the cerebrum is smooth and the olfactory lobes are still in front, though much encroached upon in the marsupial by the enlarging cerebrum. In those animals still higher in the scale of life, such as the prosimiæ (Lemurs), the cerebrum has reached yet larger proportions and complexity, and has grown still farther backwards towards the medulla, so that it hides from view a considerable portion of the cerebellum (Fig. 22); it has also grown forward, thus concealing largely the olfactory lobes. The cerebrum is no longer smooth, but has a number of simple fissures and convolutions (the higher animals have numerous complex fissures and convolutions). The lateral lobes of the cerebellum have increased relatively more than the central lobe, and the whole organ has advanced in complexity of fissures. In the higher simiæ (monkeys and apes) the cerebrum has grown so far backwards as to almost completely cover the cerebellum and medulla, and its convolutions have become much more numerous and complex. The cerebellum has also grown greatly, and its lateral lobes are now larger and more complex than the central lobe.

[Illustration: PLATE XII.—Brain of man: dorsal and side views. The cerebrum has grown so far backwards and forwards as completely to hide the other segments of the brain when looked at from the dorsal surface. From Carus’s “The Soul of Man.” By courtesy of The Open Court Publishing Company.]

Finally, in man (Plate XII), the whole brain has grown so enormously that it is three times larger than the brain of the highest simian creature. The cerebrum, especially, has increased enormously in size. It has grown not only backwards (overlapping cerebellum), upwards, and downwards on the sides, it has grown so far forwards as not only to cover the olfactory lobes, but also to project far beyond them. The cerebellum has also increased in size and complexity, especially the lateral lobes. The ideal vertical section (Fig. 23) shows diagrammatically in one figure all these stages in the evolution of the human brain through the geologic ages.

[Illustration: FIG. 23.—Ideal, vertical and sagital section, representing the ontogeny and phylogeny of the human brain. of, olfactory lobe; crf, cerebrum of fish; ol, optic lobes of fish; cbf, cerebellum of fish; m, medulla of fish; cbr, cerebellum of reptile; cbo, cerebellum of opossum; cbl, cerebellum of lemur; cbm, cerebellum of man; cr, cerebrum. Cerebrum convoluted in lemur; much more convoluted in man. Cerebellum convoluted from opossum upwards; mm, medulla of man.

Modified from Le Conte.]

It is a very interesting and instructive fact that in the development of the human brain from the fertilized ovum these same stages, which are permanent conditions in the zoölogical (taxonomic) series, are passed through by it as transient stages.

One of the earliest conditions of the human brain is that in which it presents three swellings in a serial arrangement. They are known from behind, forwards as hindbrain, midbrain, and forebrain. For our purposes it is sufficiently accurate to say that the =fœtal brain=, in developing from this early condition to a later and higher condition, differentiates the hindbrain into the _medulla_ (Fig. 24, m) and the _cerebellum_ (cb); the midbrain becomes the _optic lobes_ (ol); and the forebrain differentiates into the _thalami_ (th) and the _cerebrum_ (cr). A little later the cerebrum buds forth the olfactory lobes (of), so that the human brain will consist of six fundamental segments,—one behind the other. This is the _fish stage_ in the growth of the human brain. (Compare Fig. 24 with Fig. 19.)

[Illustration: FIG. 24.—Diagrammatic representation of the brain of a human fœtus of the third week. Representing the fish-phase in the ontogeny of the human brain. Side view. cr, cerebrum; th, thalamus; ol, optic lobes; cb, cerebellum; m, medulla. The olfactory lobes at this stage are very small and are not shown.

FIG. 25.—Dorsal view of the brain of a human fœtus of about seven weeks. Representing the reptilian phase in the ontogeny of the human brain. cr, cerebrum; th, thalami; ol, optic lobes; cb, cerebellum; m, medulla.

FIG. 26.—Side view of the brain of a human fœtus of about three months. Representing the marsupial phase in the ontogeny of the human brain. cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla. The thalami are completely, and the optic lobes partially, covered by the greatly enlarged cerebrum.]

As development proceeds the most conspicuous growth of the brain is observed in connection with the cerebrum and cerebellum. The cerebrum particularly grows relatively and actually larger and larger, but does not yet cover any portion of the optic lobes. This is the _reptile stage_, represented in Fig. 25. The cerebrum, continuing to grow, finally covers the front portion of the optic lobes. This is the _marsupial stage_, and is shown in Figs. 26 and 27. Growing further, it soon covers a greater or less portion of the cerebellum. These are the prosimian (Lemur) and simian stages. Finally it grows so far backward as to completely cover the cerebellum, and so far forward as to project much beyond the olfactory lobes. This is the human stage (Plate XII).

[Illustration: FIG. 27.—Dorsal view of the brain (and spinal cord) of a human fœtus of about three months. Representing the marsupial phase of development. cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla; bs, brachial enlargement of the spinal cord; ls, lumbar enlargement. The thalami are entirely covered and hidden from view by the cerebrum.]

In the study of the phylogeny of the brain we found that the cerebrum in fish, reptile, and lower marsupial is smooth. In the primitive primates (Lemuroidea) it is convoluted; in the simiidæ it is still more convoluted, while in man it reaches the climax of complexity in the size, number, and sinuosity of its convolutions. The object of these convolutions is to increase the surface of the cortex of the brain, the cortex being the seat of psychic phenomena. Other things being equal, the greater the amount of cortex the greater is the intelligence. During its embryonic development the human cerebrum passes also through the stage of smoothness to a convoluted condition; then through stages of increasing complexity of convolutions. Simultaneously with this advance of cerebral organization, there is an unfolding of increasing intelligence.