The Physiology of Digestion by Andrew Combe, M.B. (1837)
Part I. Chapter III. Mastication, Insalivation, and Deglution.
The Physiology of Digestion by Andrew Combe, M.D., Forth American Edition, was published By Marsch, Capen & Lyon in Boston in 1837. Andrew Combe was a physician in ordinary to their Majesties the King and Queen of the Belgians, and a Fellow of the Royal College of Physicians of Edinburgh. The third edition of this book was sold in 3000 copies. The text of this book of 310 pages will be found here, completely unchanged.
MASTICATION, INSALIVATION, AND DEGLUTITION
Mastication. - The teeth. - Teeth, being adapted to the kind of food, vary at different ages and in different animals. - Teeth classed and described. - Vitality of teeth and its advantages. - Causes of disease in teeth. - Means of protection. - Insalivation and its uses. - Gratification of taste in mastication. - Deglutition.
Having seen that a regular supply of nourishment is carefully ensured by the constantly returning impulses of appetite, we come next to examine the mode in which the food is prepared for becoming a constituent part of the animal machine, and endowed with the properties of life.
The first important step in the complicated process of digestion, is that by which the food, after being received into the mouth, is mixed with the saliva, and broken down till it becomes of a uniform pulpy consistence, fit for being easily swallowed and acted upon by the gastric juice on its arrival in the stomach. The term mastication, or chewing, is used to denote this operation; and the chief instruments by which it is performed are the teeth, the jaws, the muscles which move the jaws, the tongue, and the salivary glands. On each of these we shall offer a few observations.
The teeth vary a good deal according to the kind of food on which the animal is destined to live; but in man and the higher orders of animals they may be divided into three distinct groups; 1st, The incisor, or cutting teeth, being the eight broad and flat teeth, with a sharp cutting edge, seen in front of the upper and lower jaws, and marked I in the subjoined wood-cut, which represents one half of the lower jaw, and consequently only one fourth of the whole number of teeth. Thus we find only two incisors marked in the wood-cut, although there are eight of them in all, viz., two more are on the other side of the lower jaw, and four corresponding ones in the upper. 2nd, The cuspidati, canine or dog teeth, being the sharp-pointed, roundish-bodied teeth, four in number, on, C, in contact with each of the outer incisor teath, and called canine from being large in the dog and carnivorous in animals, and used by them for the purpose of seizing and tearing their food; and, 3rd, The molares, or grinders, B G, Twenty in number, situated at the back part of the jaw, and so called from their office being to grind or bruise the food subjected to their action (In Latin, cuspis signifies the point of a spear; canis, a dog; mola, a mill; incisor, any thing which cuts.) The term grinders, however, is sometimes restricted to the three back teeth on each side, marked G, and seen to have double roots and a broad grinding surface; and the two intervening between the and the cuspidati are styled bicuspidati, or double-speared, from bearing a greater resemblance to a double-headed canine tooth than to the other grinders.
The teeth are modified in different animals to suit their habits of life. In herbivorous animals, the canine teeth, for which they have no use, are comparatively undeveloped; whereas in carnivorous animals, which tear their prey in pieces, the canine teeth are large, powerful, and pointed, and the incisors comparatively small. In these animals they constitute what are properly called the tusks, and in some species they are of a truly formidable character. The molar, or grinding teeth, differ in like manner, according to the nature of the food. In herbivorous and granivorous animals they are large and powerful, and to increase their efficacy the lower jaw admits of considerable lateral motion in a horizontal direction; whereas, in carnivorous animals, it admits of motion only upwards and downwards, as in opening and shutting the mouth. The lateral grinding motion is very evident in ruminating animals, such as the cow, which, after having filled its stomach with provender, is generally seen to lie down and ruminate, or chew the cud, as it is called - the rumination consisting in bringing up small masses of herbage from the stomach, and submitting them to a thorough mastication or grinding between its molar teeth, before being again swallowed and digested.
From this relation between the food and the organs of mastication, naturalists can tell with certainty, by simply inspecting the teeth, on what kind of food the animal wo which they belong is intended to live; and as the teeth of man partake of the characters of those of both herbivorous and carnivorous animals, there cannot be a doubt that his diet was intended to be of a mixed kind, not confined exclusively to either the vegetable or the animal kingdom.
Hard and resisting as the teeth appear, they must still be regarded as living structures. Anatomically speaking, each tooth is divided into three parts: the fang. or root, implanted in the socket of the jawbone; the neck, or portion encircled by the gum; and the white crown, appearing above the gum, and covered with enamel.
The root of each tooth is perforated longitudinaly by a small canal, through which the bloodvessels and nerve are admitted to its central parts. From these bloodvessels the tooth derives its nourishment when growing; but they afterward almost entirely disappear. From the nerve it derives that sensibility which makes us instantly aware of the contact of bodies either too hot or too cold with the teeth; and which, when the nerve is diseased, gives rise to the racking pain of toothache.
So effectually is life maintained in the teeth by this provision of vessels and nerves, that a tooth newly extracted from the socket of a young animal, and implanted in the fleshy comp of a cock, has been found to adhere and retain its vitality; and in like manner, if, in early life, a tooth extracted by accident be immediately replaced in its socket, it will generally adhere and live.
The visible part, or crown of the tooth, is covered with a very hard, white, ivory-looking substance, called enamel, which serves to prevent it from being worn down by friction, and into which neither bloodvessels nor nerves have been observed to penetrate. Owing to this structure, the tooth can be safely exposed without sustaining damage - a privilege on which most persons will be disposed to place a higher value after having experienced the pains consequent upon injury of the nerve from a portion of the enamel being broken off.
An obvious advantage attending the vitality of the teeth is, that it enables them to accomodate themselves to the growth of the jaw and the rest of the system at the different periods of life. In early infancy, when the human being is designed to live exclusively on his mother´s milk, which of course requires no mastication, and consequently no teeth, the latter are still imperfectly formed and entirely hidden in the jaw; it is only at the end of some months that the front or cutting teeth begin to appear; and the whole set of milk, deciduous, or falling-out teeth, twenty in number, is not completed till about or after the third year. In the course of three or four years more, however, growth has advanced so far that the first set of teeth no longer fill the jaw; and they soon begin to be displaced by the second, or permanent set, the gradual development of which commences at that period of life, and is not finished till the appearence of the last four grinders, or widom-teeth, about the age of maturity.
It is a curious fact, that the infant is born with the rudiments of both sets of teeth in the jaw at the same time, although neither makes its appearence till long after birth. The permanent teeth lie in a line under the milk-teeth, and it is from their growth, causing the gradual absorption of the roots of the first teeth, that the latter no longer retain their hold of the jaw, but drop out as soon as the others are ready to protrude. In the preceding wood-cut, the situation of the permanent teeth before they emerge from the jaw is rudely represented at A, where the outer surface of the jaw-bone has been removed on purpose to show the appearance of the roots. But nothing of this kind is to be found in the adult jaw, the parts marked A being inserted in the plate merely to illustrate what was once the position which the permanent teeth occupied.
The changes in the condition of the teeth, it may be remarked in the passing, indicate clearly what species of food nature has intended for us at different ages. In early infancy, when no teeth exist, the mother´s milk is the only nutriment required; and in proportion as the teeth begin to appear, a small addition of soft farinaceous food, prepared with milk, may be made with propriety, and gradually increased. But it is impossible to look at the small jaw, moderate muscle, and imperfect teeth of early life, without perceiving that only the mildest kinds and forms of animal food are yet admissible, and that the diet ought to consist essentially of soft and unirritating materials. It is not till the permanent teeth have appeared that a full proportion of the ordinary kinds of butcher-meat becomes either beneficial or safe.
The teeth, being living parts, and at the same time endowed with a mechanical function, are liable to injury in both capacities. Being composed chiefly of earthy matter, such as phosphate and carbonate of lime, the contact of strong acids decomposes their substance, and leads to their rapid decay. Hence, the whiteness produced by acid tooth-powders and washes is not less deceitful than ruinous in its consequences; and hence also great caution is necessary in swallowing the acid drops frequently prescribed by the physician, which ought never to be allowed to come in contact with the teeth.
Being constantly moistened with saliva, the teeth have a tendency to become incrusted with the tartar, or earthy matter, which it contains in solution, and which is separated from it partly by the evaporation of the more fluid constituents in breathing, and partly by chymical decomposition. As this incrustation not only destroys the beauty of the teeth, but also promotes their decay, it becomes an object of care to remove it as soon as it is formed; and the most effectual mode of doing so is to brush the teeth regularly twice a day - especially in the morning, when the quantity is greatest - with a brush dipped in soft water, till every particle is removed. The addition of any soft impalpable powder will assist in the effect; but nothing capable of acting chymically on the teeth, or of injuring them by friction, ought ever to be resorted to. Washing the mouth after every meal is also a good preservative.
When the teeth are not used for a time, and when digestion is impaired, the quantity of tartar which accumulates on them is very great. Hence they are always most incrusted in the morning, and in fevers and other affections where no food is taken, and the stomach is at the same time disordered. I have seen one instance in which a thick crust of tartar was removed by a dentist in the belief of its being a diseased tooth - the tooth itself on which it was formed being left in the jaw perfectly sound.
When the tartar is not duly removed, its presence injures the teeth, irritates the gum, and generally leads, sooner or later, to considerable suffering. The regular washing and brushing above mentioned ought, therefore, to be sedulously practised at every period of life, and taught as a duty to the young. When digestion is very vigorous, the health good, and the diet plain, and containing a full proportion of vegetable matter, the deposition of tartar seems to be diminished, and the teeth to be naturally of a purer white. Many rustics and savages thus possess teeth which would be envied in a town.
When digestion is impaired, and acidity prevails in the stomach, the mucous secretions in the mouth also become altered in character, and by their incessant contact injure and even destroy the teeth. From this cause we often see the teeth in young people in a state of complete decay. They are in reality the subjects of chymical decomposition, and eaten away by the morbid secretions of the mouth; and hence, in such cases, we generally find the individual complaining of heat and soreness of the tongue, gums, and mouth, and occasionally of the teeth being "set on edge."
Considered as living parts, the teeth require some additional care. In that capacity they are exceedingly apt to suffer from sudden changes of temperature. Being from their solidity rapid conductors of heat, their internal nerve speedily becomes affected by the alterations of temperature to which they are daily exposed, both in taking food, and in the change from a warm to a cold athmosphere. It is a not uncommon practice, for example, to take a glass of cold wine or water immediately after finishing a plateful of very hot soup; and it is quite usual to take tea and coffee, and every kind of meat, as ot as they can possibly be swallowed - than which practices it would be difficult to imagine any thing more hurtful to the teeth.
For the same reason, in going out at night from a warm room to the cold air, it is desirable to protect the teeth from the influence of the sudden change, by breathing through two or three folds of a silk handkerchief, or through a woollen comforter. When the teeth and lower part of the face are left exposed in such circumstances, rheumatism and toothache not unfrequently ensue from the direct impression of the cold air upon parts rendered more susceptible by the preceding heat.
The great source of injury to the teeth, however, both in childhood and in mature age, is disordered digestion. If hte health be good, and the stomach perform its functions with vigour, the teeth will resist much exposure without sustaining injury. But if these conditions fail, they will rarely continue long unscathed.
It is almost always from the latter cause that, in infancy, teething so often gives rise to serious constitutional disorder.
Something more, however, than the mere action og the teeth and jaws, is required to prepare the morsel for being swallowed. If we take a bit of dry bisquit or mealy potato into the mouth, and attempt to masticate it, we encounter at first no small difficulty from the stiffness and resistance of the dry mass, and feel instinctively that it would be in vain to attempt to swallow it, until moistened either by continued mastication or by the admixture of fluid from without. In ordinary states of the system, accordingly, a fluid called saliva, or spittle, is copiously secreted and poured into the mouth for this very purpose; and the process by which its due admixture with the contents of the mouth is accomplished, is called the insalivation of the food.
To provide this necessary fluid, and to connect its supplies directly with the process of mastication to which it is subservient, several glands for its secretion have been placed in the immediate neighbourhood of the mouth and jaws, in such a way that the latter cannot be opened and shut without affording them a stimulus, and still farther increasing the secretion which the presence of the morsel is itself sufficient to begin. From this arrangement it follows, that the more perfectly mastication is performed, the more thoroughly does the morsel become impregnated with the salivary fluid, and the better fitted is it rendered for subsequent deglutition and digestion.
The apparatus of mastication varies according to the kind of food on which the animal is destined to live; but in the higher orders of animals, it consists essentially of the parts already mentioned. In some animals, however, which live on soft gelatinous food - as the whale - no teeth are to be found, because their peculiar power is not required. In others - as the granivorous, or grain-eating birds - the grinding or triturating process is effected not in the mouth, but in the gizzard, where the food (mixed with gravel, which the animal is instinctively impelled to swallow for the purpose) is effectually bruised and softened down by the strong muscles which constitute the greater part of its substance. In these instance the gravel is the grinding instrument, and without its presence digestion cannot be carried on, any more than it could in man without the agency of teeth.
The degree of mastication required varies also, according to the mode of life of the animal, and the digestibility of its food. Animal food, for example, being easy of digestion, requires less mastication than vegetable food, which is more difficult. This is so much the case, that most animals which live on fresh vegetable matter spend half their waking hours in ruminating, or re-masticating the food, which they have already cropped and stored up for the purpose in one of their four stomachs. To this necessary act in them, Providence seems to have attached a high degree of gratification, for the very purpose of ensuring its regular performance.
Man, being naturally omnivorous, or adapted for the digestion of both animal and vegetable substances, holds, as it were, an intermediate place in regard to the rapidity of mastication. He neither is obliged to ruminate like the cow, nor can beneficially bolt his food with the rapidity displayed by beasts of prey. His object is merely to reduce the alimentary mass to a soft and pulpy consistence, and digestion is promoted or retarded in exact proportion as he approaches or falls short of this point. Hasty mastication is consequently injurious, because it prevents the food from being sufficiently broken down and impregnated with saliva; and the more uncommon error of protracted mastication is also injurious, owing to the undue dilution which the mass sustains from the overflow of the salivary secretion.
Due mastication being thus essential to healthy digestion, the Creator, as if to ensure its being adequately performed, has kindly so arranged, that the very act of mastication should lead to the gratification of taste - the mouth being the seat of that sensation. That this gratification of taste was intended, becomes obvious, when we reflect that, even in eating, nature makes it our interest to give attention to the process in which we are, for the time, engaged. It is well known, for example, that when food is presented to a hungry man, whose mind is concentrated on the indulgence of his appetite, the salvia begins to flow unbidden, and what he eats is consumed wiht a peculiar relish. Whereas, if food be presented to and individual who has fasted equally long, but whose soul is absorbed in some great undertaking or deep emotion, it will be swallowed almost without mastication, and without sufficient admixture with the saliva - now deficient in quantity - and consequently lie on the stomach for hours unchanged. A certain degree of attention to taste, and to the pleasures of appetite, is, therefore, both reasonable and beneficial; and it is only when these are abused that we oppose the intentions of nature.
From the existence of this interntional relation between mastication and the salivary secretion, the latter is always most copious in those creatures whose food requires continued mastication. In ruminating animals, accordingly, the salivary glands are numerous and of great size, while they are at the same time so situated, that the play of the muscles in the act of rumination communicates to them a proportionate stimulus. In those, again, which do not masticate at all, but swallow their food entire, there is scarcely any salivary secretion, and the glands appropriated to it are very small. Birds, and many fishes and reptiles, belong to the latter class.
From the foregoing explanation of the object and conditions of mastication, the reason will be apparent why fluids do not require to undergo that process, and also why dry mealy substances stand in need of protracted chewing before they can be easily swallowed. When hot spicy food is taken into the mouth, the secretion of salvia is immensely increased, obviously for the purpose of diluting the excess of stimulant before it shall be allowed to reach the stomach. But when the food is of a mild and unirritating quality, much dilution is unnecessary, and the secretion is accordingly moderate.
William Beaumont, 1785-1853. This illustration is from a later source.
The chief purpose of mastication, then, is evidently the minute division of the aliment, so as to admit of its being easily acted upon by the gastric juice when received into the stomach. Dr. Beaumont, however, seems to me to go too far in inferring, that "if the materia alimentaria could be introduced into the stomach in a finely divided state, the operations of mastication, insalivation, and deglutition, would not be necessary." It would require a more extensive range of experiments than that which he has made, to prove that "aliment is as well digested and assimilated, and allays the sensation of hunger as perfectly, when introduced directly into the stomach (through an opening in the side) in a proper state of division, as when the usual previous steps have been taken." (Experiments and Observations on the Gastric Juice and the Physiology of Digestion, by William Beaumont, M.D. Plattsburg, 1833, p 67.) It is quite true that mastication and deglutition are chiefly mechanical prosesses; but it is difficult to believe that so much care would have been taken to provide a proper supply of a fluid of a constant and peculiar character like saliva, if water were capable of answering the purpose as well, and if saliva were useful only in lubricating the food. There subsists, moreover, between the sense of taste and the power of digestion, a certain relation, which renders it more than probable that the active gratification of the former during mastication is favourable to the production and flow of nervous energy towards the stomach, and consequently in so far conducive to the healthy performance of digestion, that even in that point of view insalivation could not easily be dispensed with. Dr. Beaumont´s experiments, however, abundantly demonstrate that Montégre, and those who, along with him, consider the saliva as the principal agent in digestion, have not a shadow of foundation for their opinion.
When unmasticated food is introduced into the stomach, the gastric juice acts only upon its surface, and other purely chymical changes sometimes commence in its substance before its digestion can be effected. Hence often arise, especially in children, those pains and troubles, that nausea and acidity, consequent on the continued presence of undigested aliment in the stomach. By a peculiarity of constitution, however, the stomach will not long retain food which it cannot dissolve. After a number of hours, - varying, according to the state of health, from one or two to ten, or even twenty, - it is either rejected by vomiting, or transmitted unchanged to the intestine, where its presence irritates and gives rise to colic, flatulence, bowel-complaints, and, in delicate children, not unfrequently to convulsions. Hence another proof of the importance of slow and deliberate mastication.
As soon as the morsel has been thoroughly masticated and impregnated with saliva, it is ready for transmission to the stomach. To this part of the process the term deglutition, or swallowing, is applied.
Immediately at the back part of the mouth several passages present themselves, leading in different directions - one upwards and forward into the nose, another downwards and in front into the windpipe, and a third downwards and behind into the oesophagus, or gullet, and stomach. The last is the passage taken by the food, and the violent coughing and occasional suffocation induced when it accidentally passes into the windpipe, are but a specimen of the serious evils which would be continually occuring if some provision were not made to obviate the danger, while the rarity with which such accidents actually happen, proves the almost unfailing efficacy of that which has been devised.
The passage of the food into the nostrils is prevented by the interposition of a moveable fleshy fold or valve hanging down from the palate, and visible at the back part of the mouth; this, in the act of swallowing, is stretched backwards so as to extend to the back part of the throat, and thus entirely shut up the opening into the nostrils. The passage into the windpipe, again, is protected by a cartilaginous lid or covering called epiglottis (from epi, upon, and glottis, the tongue), which projects backwards from the root of the tongue, and conducts the morsel over the glottis or opening of the windpipe. The epiglottis, however, is greatly assisted in this operation by that rising upwards and forwards of the gullet and windpipe to meet the morsel, of which we are concious in the act of swallowing, and the effect of which is in some degree to hide the glottis under the backward projection of the root of the tongue, and allow the morsel to drop past into the gullet.
Once fairly in the gullet, the course of the food into the stomach is easy enough. The gullet is simply a round tube, made up of two rows of muscular or fleshy fibres, the one longitudinal, and the other transverse and circular, with a soft, moist lining membrane to facilitate the transmission of it contents. When the morsel is introduced, its upper part contracts involuntarily, and pushes the mass downwards; the portion now reached contracts in its turn, and propels it farther; and so on in sucsession till it arrives at the stomach.
Deglution, or swallowing, is thus a more complicated operation than at first sight it appears to be. On looking at any person eating, one is apt to think that the morsel passes along the gullet into the stomach by its own weight; but we speedily perceive the error when we recollect taht, in the horse and the cow for example, the mouth is on a level with the ground when feeding, and that the morsel is consequently propelled upwards into the stomach against its own gravity. It is well known also, and often made a matter of public exhibition, that a man can swallow even liquids when standing on the crown of his head, with the natural position of the stomach reversed.
Deglutition is easier and quicker when the appetite is keen, and the alimentary bolus or morsel is moist and properly softened. It is slow and difficult when the morsel is dry and mealy, and the appetite nauseated. In vomiting, the action of the muskucar fibres is inverted, or proceeds from the lower end of the gullet towards the mouth; and hence the object is carried upwards instead of downwards, as in the natural order.