Part 8

(_b_) The pterygoid muscles, springing as they do from the internal pterygoid plates, must necessarily be in close relation with the naso-pharynx, especially the internal pair, which take their origin from the internal aspect of the internal plates. I would further point out that the external pair, although they diverge from the naso-pharynx on their way to the mandibular condyles, yet remain on a level with that cavity. This close relation of the pterygoids to the naso-pharynx is, if I mistake not, of great importance in relation to the etiology of “adenoids.”

(_c_) Of the two pairs of pterygoids the external pair pass in the more outward direction, forming with the sagittal plane of the head an angle of 45° (see Figs. 1 and 2). In consequence of this direction they tend by their contraction to pull the pterygoid plates and posterior parts of the maxilla away from the sagittal plane of the head, and thus to secure the normal width of the posterior nares. It is these muscles which bring about the lateral movements of the mandible, causing the lower teeth to move laterally and sagittally across the upper, the food being in this way far more effectually ground than by a mere vertical pressure of the teeth against one another. These lateral movements are, as we shall see, less pronounced among the moderns than among primitive peoples.

[Illustration: FIGURE 2.--Portion of horizontal section of head about an inch below the condyles of the lower jaw. The outward direction of the external pterygoids is well shown; also the close relation of the levatores and tensores palati with the internal pterygoids.]

_The influence of the contraction of the masticatory muscles on the local circulation of blood and lymph._--When a muscle is at rest the blood flows sluggishly through it, while there is a complete, or all but complete, stagnation of the lymph current; if a lymphatic trunk of a limb at rest be cut no lymph escapes from it. Rhythmic muscle contractions, however, stimulate the flow both of blood and lymph (_a_), in the contracting muscles themselves and (_b_) in the neighbouring parts. (_a_) Not only are the muscle arteries dilated during rhythmic contractions, but the blood is vigorously squirted out of the muscle veins, so that much more blood flows through a muscle during its rhythmic contraction than during rest. The flow of lymph is even more markedly stimulated,--this fluid, which, while the muscle is at rest, is stagnant or all but so, being during contraction driven

## actively along the lymphatic trunks. (_b_) How greatly rhythmic muscle

contractions influence the circulation of fluids in the neighbouring parts is shown by the flushing of the skin and the swelling of the soft parts generally of a limb which is being exercised. We thus see how profoundly the exercise of the masticatory muscles--and among these we must not forget to include the tongue--influences not only their own nutrition but that of the important structures adjacent to them--that is to say, of the jaw-bones, salivary glands, buccal mucous membrane, soft palate, faucial tonsils, pharynx, and naso-pharynx, as well as of the nasal cavities and their accessory sinuses. All these parts are during mastication copiously flushed with blood and lymph, from which it is evident that efficient mastication must stimulate their nutrition and favour their proper development. Hence, in one who has from childhood upwards been accustomed to masticate efficiently, we generally find these parts well developed, the jaws large and shapely, the teeth regular and straight, the tongue and salivary glands large, the nasal and naso-pharyngeal passages spacious, and the mucous membrane of the buccal and adjoining cavities healthy.

_Influence of mastication on the jaw-bones._--It is well known that the size of a bone is largely determined by the degree to which the muscles attached to it are exercised. That the jaws do not grow to their normal size, if not adequately exercised during their period of growth, is strikingly shown by the overcrowding of the teeth, which takes place in those brought up on soft foods, and this even though there be no contraction of the jaws resulting from mouth-breathing. The dependence of the size of the jaws upon the degree to which they are exercised is also shown by the smallness of the modern jaw, as compared with that of primitive peoples, a difference which, as we shall see, is in part congenital and in part due to the comparative disuse of the former. Mastication influences not only the size but also the shape of the jaws (_a_), through its influence on the size of the tongue, which by pressing against the teeth tends, as Sim Wallace has shown, to expand the jaws; (_b_) by the pressure of opposing teeth against one another, which has a similar effect; and (_c_) by the outward pull of the pterygoids, which tends to widen the maxilla posteriorly and to broaden the posterior nares.

_Influence of mastication on the teeth._--The teeth being developed within the jaw-bones and remaining, even after eruption, in close anatomical and physiological association with them, must necessarily share in their nutritive tendencies. If these bones are efficiently exercised during the formation of the teeth--and my remarks apply especially to the permanent set--the tooth-germs will be abundantly flushed with blood, while the ample growth of the jaws themselves will provide the germs with plenty of room in which to grow and to develop, and the more perfect their growth and development the more resistant should we expect them to be to the ravages of caries. Who can contemplate the jaw-bones of a six-years-old child, dissected so as to display all the imbedded teeth, without being assured of the effect of mastication upon dental development? Fifty-two teeth meet the view: the whole region from the orbital rims to the inferior border of the mandible is literally paved with them, and I can hardly doubt that they collectively weigh more than the bone in which they are imbedded. Surely no one can examine such a dissection without being convinced of the urgent necessity, if the teeth are to grow and to develop normally, of giving the child’s jaws from infancy onwards plenty of work to do.

The ample development of the jaws, which efficient mastication brings about, has a further beneficial effect as regards the teeth, in that it enables them to take up their proper places in the alveolar ridges, thus securing all the advantages of a good “bite.” These I now proceed to consider. The teeth during mastication, and especially when the bite is good and the food of a kind necessitating vigorous and sustained mastication, are made to move in their sockets both vertically and horizontally; the effect of this is to stimulate the circulation in the tooth-pulp, the alveolar periosteum (and hence also in the cementum and alveolar walls which are supplied by it), and the circumjacent mucous membrane of the gum. All this makes for the health of the teeth; not only does it promote the nutrition of the tooth itself and of its bony socket, thus maintaining a firm dental setting, but it also tends to secure a healthy environment for the exposed part of the tooth--that part, namely, wherein caries begins--by maintaining a healthy state of the surrounding and, indeed, of the entire buccal mucous membrane, as well as of the various secretions which bathe the mouth. Wherefore it is not surprising to find that those who masticate efficiently suffer much less from dental caries and its complications (such as abscess at the root) and disease of the periodontal membrane (_e.g._, pyorrhœa alveolaris and loosening of the teeth) than those who are accustomed to bolt their food.

A few words as to the influence of mastication in wearing down the teeth. In those races which masticate vigorously the teeth in quite early adult life show signs of wearing away, while in later life it is quite common for the biting surfaces to be worn flat; sometimes the crown of the molars is worn away so that its surface shelves downwards and inwards and not infrequently it is concave, having a scooped-out appearance; often the dentine is exposed in this way; and yet among many hundreds of skulls examined I do not remember to have seen one single case where caries has started on the biting surface thus worn down.

I had always attributed this wearing down of the teeth to the friction of coarse food against them. Primitive races eat coarse vegetable food, which frequently contains grit, and this doubtless helps to grind the teeth down, but they may be markedly ground down even in those living on soft food, and in such cases the grinding away can obviously only be due to the friction of opposing teeth against one another. I, indeed, believe this to be the essential cause of the phenomenon, both in civilised races living on soft food and in primitive races whose coarse food necessitates prolonged and vigorous mastication and a corresponding amount of attrition between the biting surfaces of opposing teeth. In order that this attrition may occur two things are requisite: the upper and lower teeth must be well opposed--there must be a good bite--and mastication must be vigorous and of the right kind. Mere vertical pressure of the teeth against one another will not wear away the opposing surfaces; there must be friction of these surfaces against one another--a transverse and sagittal movement of the lower teeth against the upper by means of the pterygoids. Mainly to this do I attribute the marked wearing down of the teeth observed in primitive peoples, and I am gratified to know that so competent an authority on dental pathology as Sim Wallace is a convert to this view.

That all the teeth may be worn down just as we observe in primitive people, even in those who have lived all their lives on the ordinary fare of the moderns, is proved by a case I have under observation. It is that of a man in his fiftieth year, who was brought up in Belgium but who has resided in London for the last thirty years. When he came to my out-patient room I was not a little surprised to find that all his teeth were sound--a very unusual occurrence, I need hardly say, among the London poor at his age. In seeking for an explanation I elicited the fact that he was unable to swallow his food without chewing it very thoroughly, and on giving him a moderate-sized piece of bread, with the request that he should chew it in the ordinary way, I found that he subjected it to one hundred and twenty separate bites before swallowing it, and in the steady, deliberate way he went to work and in his extensive lateral movements of the mandible he reminded one for all the world of a cow chewing its cud. The temporals and masseters of this man are enormous, and the like is no doubt true of the pterygoids; he has well-developed nasal passages, has never suffered from nasal obstruction, while his buccal mucous membrane is unusually healthy for one of his years and circumstances. May we not attribute this healthy state of the mouth, teeth, and nose to the good effects upon them of efficient chewing? Here is a man who has lived for thirty years in London on the same kind of food as the average poor Londoner, but instead of finding his mouth full of carious, tartar-coated teeth, and spongy, receding, pus-exuding gums, we find thirty-two sound teeth firmly set in healthy gums and all but devoid of tartar.

A word as to the wearing down of the teeth in the anthropoid apes. In this respect the gorilla differs markedly from the orang and the chimpanzee. In all the skulls of these latter which I have examined the teeth show signs of wearing away, while I have found the teeth of the gorilla, with the exception of the tusk-like canines, but little worn. From this we should expect the latter animal to be mainly carnivorous, and the orang and chimpanzee to be largely herbivorous.

SECTION II. From _London Lancet_, July 18, 1903

CHANGES WHICH THE JAWS AND TEETH OF MAN HAVE UNDERGONE DURING MAN’S EVOLUTION FROM HIS ANTHROPOID ANCESTORS

During man’s progress upwards from the anthropoid his diet has undergone a progressive change, and a parallel adaptation has taken place in his jaws and teeth. Dietetically considered, we may divide his evolutionary career into the following epochs[8]: (1) the anthropoid stage; (2) the pre-cooking human stage; (3) the cooking pre-agricultural stage; (4) the early agricultural stage; and (5) the late agricultural stage.

1. _The anthropoid stage._--The diet of man’s anthropoid ancestors was probably much the same as is that of existing anthropoid apes; it consisted, namely, of raw vegetable and animal food, necessitating a vigorous use of the maxillary apparatus. This latter, we may assume, was of the type belonging to the anthropoids--_i. e._, the jaws were massive and markedly prognathic; the denture was the same as it is in existing man, but the teeth were larger, especially the upper canines, which served as weapons of offence and defence; the third molars (the wisdom teeth) were as large as the other molars and were provided with three fangs, and there was an ample portion of alveolar ridge behind them; there was no chin. No doubt the massiveness and the marked prognathism, which characterised the jaws at this stage, served other ends than that of mastication; it is obvious that projecting jaws and teeth are much more effectual for seizing and lacerating prey than are the orthognathic jaws of modern man.

2. _The pre-cooking human stage_ extends from the time man’s ancestors first assumed the human form till they learned to apply fire in the preparation of their food. During all this period the jaws and teeth were probably used as much, or almost as much, for mastication as during the anthropoid stage; raw animal food had to be torn from the bones, the latter had to be crunched, while the bulk of the raw vegetable food needed then no less than it needs now prolonged and vigorous mastication in order to liberate the starch and other nutritive ingredients from their undigestible cellulose envelopes.[9] Nevertheless, the jaws and teeth underwent considerable change during this period, for not only were they with every advance in intelligence called less and less into requisition for purposes of offence and defence, but the jaws, at least, became materially modified in correlation with the expanding cranial cavity and in connection with the assumption of the erect posture. It is, I think, rather for these reasons than in consequence of alterations in the nature of the food that the masticatory apparatus now gradually lost its more bestial aspect and assumed an essentially human type, becoming towards the close of the period much the same as may be observed among the most primitive peoples now living.

3. _The pre-agricultural cooking period._--The characters of the maxillary apparatus belonging to this period are still available for study, the aboriginal Australians, the Bushmen, Negritos, and many Esquimaux not having yet emerged from it. So far as mastication is concerned, cooking influences vegetable far more than animal food, for it not only softens it but by rupturing the undigestible cellulose chambers and liberating their contents relieves mastication of one of its essential functions. Wherefore, with the advent of cooking, man’s jaws and teeth began to get smaller, and they have continued to diminish in size up to the present time. No great diminution, however, took place at first, inasmuch as the diet still continued to be largely animal (and prior to the use of knives and forks such food had to be torn by the teeth), while the coarse vegetable food of this date, even when cooked, still needed laborious mastication. The chief differences between the maxillary apparatus of this early cooking age as compared with that of the present day are as follows: the jaws of the earlier period--_e. g._, in the aboriginal Australian--are more massive, and their sagittal diameter is greater, giving rise to decided prognathism, the teeth for the most part are larger and stronger, the third molars being nearly, if not quite, as big as the other molars, and provided with three fangs, while there is a considerable portion of alveolar ridge behind them. The third molars, however, show a decided tendency to be smaller than the rest, and the alveolar ridge behind them is less marked than in the previous period, features, I doubt not, attributable to the influence of cooking in diminishing mastication. Dental caries is rare and is chiefly met with in the third molars.

4. _The early agricultural age._--All the existing primitive races which have attained to the cultivation of the soil may be regarded as belonging to this period. Previously to it man was mainly carnivorous, owing to the comparatively limited quantity of vegetable food available, so long as the supply was left to nature alone; but when by cultivation this supply was increased and, at the same time, rendered more constant and certain, he gradually became less carnivorous and more vegetarian in his diet. The result of agriculture, however, is not only to increase the supply of vegetable food, but to diminish its fibrous, cellulosic ingredients, and thus to render it more easily masticated. Hence at this stage we find the maxillary apparatus becoming smaller than in the previous period, although the difference as shown--_e. g._, by the examinations of the skulls of the African negroes and the Melanesians--is less pronounced than we might perhaps have anticipated; prognathism is not so decided, the jaws are smaller, also the teeth, especially the third molars, which now for the first time show a tendency to be furnished with two instead of three fangs, while the alveolar ridge behind them is distinctly shorter than in the preceding period. Dental caries, hitherto rare, now becomes more frequent.

5. _The late agricultural period._--A mid-agricultural period might be described, but I shall take no account of it here, but pass on to a consideration of the late agricultural period--that, namely, in which we ourselves live. The chief characteristic of the food of this period is its softness. Cooked animal food requires, indeed, more mastication than raw, but the vegetable food of to-day, owing to the combined effects of improved agriculture, and skilful milling and cooking, is so soft as to excite comparatively little mastication. The present may, in fact, be described as the _age of pap_. Hence the jaws and teeth are now called upon to perform far less work than in any earlier stage of our evolution, and there has taken place in consequence a great diminution in their size, more especially in the size of the jaws, so that there is now often no room for the teeth to take up their normal positions, and there is generally a complete absence of alveolar ridge behind the last molars. The latter are, moreover, apt to be very small or even absent, while dental caries is alarmingly frequent.

It will thus be seen that from the period of the anthropoids to the present time, a progressive change in the size and shape of the jaws and teeth has been taking place, a change which is to be explained by (1) the cessation of the need for using them for offensive and defensive purposes; (2) the growing capacity of the cranium and the assumption of the erect position; (3) the progressive alteration in man’s diet; and probably also (4) considerations of beauty. The first three factors have operated through natural selection, the last through sexual selection, which has come into play, I would suggest, chiefly within recent times. Probably the most pronounced change which has taken place in the jaws during the agricultural periods has been the suppression of prognathism which, in the woman especially, is very unsightly, and tends to diminish the likelihood of marriage.

INSTANCES OF THE VIGOROUS USE TO WHICH THE JAWS AND TEETH ARE PUT AMONG EXISTING PRIMITIVE PEOPLES

A study of existing primitive peoples brings forcibly home to the mind how laboriously the jaws and teeth of our primitive ancestors were used. I have already shown how in pre-agricultural and early agricultural times the nature of the food compelled a sustained and vigorous exercise of these structures, and I wish here only to refer to a few specific and peculiar instances of laborious mastication exercised by primitive races now or recently living.[10] Among some of these mastication has been promoted almost to the position of an industrial art.

_The chewing of very tough substances in order to extract therefrom liquid or nourishment._--The recently extinct Tasmanians included among their articles of diet a species of sea-weed which, even when cooked, was so tough as to require long-sustained mastication in order to extract its nutrient elements. The Indians of North California chew kelp, which is “as tough as white leather” (_i. e._, leather dressed with alum). “A young fellow with good teeth will masticate a piece of it a whole day.” Again Featherman[11] tells how when the Bushmen are short of food in the winter they steep an old dried gnu-skin in water and, having rubbed off the hair, boil it, and proceed to gnaw the tough morsel until their very jaws ache. The Modoc Indians are said to munch the raw kais root all day long.[12] Among the Esquimaux it is a universal custom to chew the raw skin of the whale, the porpoise, and the seal for the blubber it contains, and the skin being as tough as india-rubber, it requires, as may be imagined, a good deal of chewing. The Lower Californians also chew deer-skin and ox-skin (Bayert). The more southern Esquimaux, according to Nansen, preserve the stalks of _angelica_ by steeping them in a mixture of chewed blubber and saliva. Finally, I may refer to the habit of chewing the sugar-cane, a practice which is prevalent among the natives in all countries where the cane grows, and affords, it need scarcely be said, abundant exercise for the jaws and teeth.

_Mastication in the preparation of beverages._--I find that among widely separated aboriginal peoples chewing is resorted to in the preparation of beverages, both intoxicating and non-intoxicating. The Gran Chaco Indians make an intoxicating drink by chewing the algarroba bean and then spitting into a receptacle. In other parts of South America berries are chewed with the same object. In some of the Pacific Islands boys and girls with good teeth are selected to chew a root (kava), from which they then prepare a drink. In New Guinea drinks are similarly prepared from roots. Boiled cassava root is chewed by the Indians of Nicaragua for the same purpose. In British Guiana the natives make a drink by adding chewed maize and saliva to sweet potato, maize, and sugar-cane. The Indians in Honduras, after steeping cassava cake or carbonised bread in hot water, chew a portion and mix it with the rest.