Digestion, is a function common to all animals, by which foreign substances, introduced into their bodies, and submitted to the action of certain organs, change their qualities and form a new compound, fit for the purposes of nourishment and growth. Animals alone are provided with digestive organs; all, from man to the polype, have an alimentary cavity, and its existence is, therefore, an essential character of animals. The loss which the body sustains, in performing the various actions that take place in the living animal machine, is supplied by means of the food. Hunger and thirst admonish us of the wants of our frame, and the pleasures of the palate are a no less strong inducement to the procuring and taking of food.

The cause of hunger has been placed in the mutual attrition of the ruge of the empty stomach, in the irritation produced by the gastric juice, &c. Perhaps it may be derived more justly from a sympathy between the stomach and the body at large. For when, in diseases of the pylorus, the food cannot be transmitted into the intestines, and does not therefore enter the system, great hunger is experienced, even although the stomach may be filled. Much depends on habit, and on the operations of mental causes: hunger is felt at the usual periods of our repasts; and, if it be not then removed by eating, will often cease spontaneously. The man of letters, absorbed in meditation, often forgets the natural wants of his body. Whatever diminishes the sensibility of the stomach makes hunger more tolerable. Thus, the Indian and Turkish fanatics (Mollahs and Fakirs) are said to support their long fasts by the habitual use of opium. Thirst seems to consist more in a very troublesome dryness of the fauces and œsophagus, and in a particular irritation of these parts, from the admixture of acrid, and particularly saline, matters with the food. The necessity of obeying both these calls varies according to the age, constitution, and particularly the habits of individuals: yet we may state, on the whole, that a healthy adult could not abstain from food for a whole day without bringing on considerable weakness; and that this abstinence could not be continued to the eighth day without the most imminent risk of life. Continued abstinence dimiBishes the weight of the body to a degree which becomes sensible in twentyfour hours, causes absorption of fat, great prostration of strength, increased sensi

bility, with watchfulness, and a most painful dragging at the epigastric region. Hunger is more speedily fatal in proportion to the youth and strength of the individual. Thus, the wretched father, whose dreadful history is immortalized by Dante, shut with his children in a dungeon, perished last, on the eighth day of confinement, after witnessing the death of his four sons, amid the convulsions of rage and cries of despair. We meet with a large collection of examples of long abstinence in the great work of Haller; but they do not seem to possess, in every instance, the requisite authenticity. Many of the subjects were weak and delicate women, living in a state of almost complete inaction, where the powers of life, almost extinct, were only evinced by a very low pulse and respiration repeated at long intervals. They might be compared to hybernating animals, where the waste, occasioned by the functions of active life, does not take place, and where consequently the usual supplies cannot be needed. Although the admonitions of thirst are very imperious, yet drink does not seem so necessary to life and health as solid food. The mouse, quail, parrot, and several other warmblooded animals, never drink, and instances have been known in the human subject. Thirst always becomes greater when any watery secretions are much augmented, as in dropsy, and particularly in diabetes.

In the dispute, whether man be naturally carnivorous, or herbivorous, we are inclined to suppose that truth lies on neither side. That the structure both of the teeth and intestines, as well as of the joint of the lower jaw, occupies a middle place between the two just mentioned, and constitutes him an omnivorous animal. This, indeed, seems to follow necessarily from the unlimited extent of his habitation; he can dwell in every country and climate of the globe; and makes use, in various situations, of every variety of alimentary matter furnished by the animal and vegetable kingdoms.

The food of man, and probably of every animal, is derived from organized matter. Nothing seems capable of furnishing nourishment that has not lived: the mineral kingdom, indeed, supplies some articles of seasoning, which are mixed with our food, and various medicines and poisons, which do not seem to be nutritious.

As man on the one hand is a most truly omnivorous animal, and capable of con

verting into nourishment almost every production of the animal and vegetable kingdoms, so on the other side he may continue strong and healthy although using one, and that a very simple kind of aliment. A woman, whose case is related in the memoirs of the Medical Society of Edinburgh, lived on whey for fifty years. Many men live only on certain vegetables, as potatoes, chesnuts, dates, &c. Some wandering Moors, according to Adamson, live almost entirely on gum Senegal. Fish is the only food of numerous uncivilized tribes on different coasts: and flesh of others. Some barbarous hordes still eat raw meat, and even the human body sometimes serves them as a repast. In several islands between the tropics, particularly in the South Sea, there is no fresh water, and the milk of the cocoa-nut is used instead: various other singular facts relating to the food and drink of man, might be collected here, showing very clearly that he is an omnivorous animal.

Whatever be the diversity of food, the action of our organs always separates the same nutritive principal from it: in fact, let the diet be totally vegetable, or totally animal, the peculiar composition of our organs does not alter, an evident proof that the matter we extract from aliment to appropriate to ourselves, is always alike.

It has been a matter of dispute, whether pure water furnish any nourishment, or be a mere diluent.

We have nothing further to say concerning the processes of mastication and deglutition, than what the reader will find in the article ANATOMY, under the head of "Organs of Mastication and Degutition."

Salivary Secretion. This has been estimated by Nuck at the quantity of one pint in twenty-four hours. Although it probably goes on, to a certain degree, at all times, yet it is more copious when we take food and the augmentation of quantity arises partly from stimulus, partly from mechanical pressure. When any acrid matter is taken into the mouth, an increased flow of saliva is produced; and this may also follow the mere sight of food, and hence has arisen the well-known expression of the "mouth watering."

All the salivary glands are so situated, that the motions of the jaw, and other instruments of mastication, necessarily subject them to considerable pressure, by which their secretory tubes are evacuated, and new secretion promoted.

The saliva is conveyed into the mouth by the contractile power of the salivary ducts, which, in some rare instances, are said to have projected it even from the cavity of the mouth. The great number of vessels and nerves, which belong to and are placed near these glands, correspond to the copious supplies of fluid which they .furnish.

Besides the simple water furnished by the true salivary glands, the mucous follicles, which abound on the surface of the mouth, supply a considerable proportion of that fluid, to be mixed with the food. These additions being, by means of mastication, intimately blended with the food, not only reduce it to a soft pultaceous mass, more fit for the process of deglutition, but also bring it into a state of convenient preparation for the subsequent process of digestion and assimilation. In this point of view, mastication is very important, as we may observe from the ill effects which ensue when the loss of the teeth renders it imperfect in old persons.

For the chemical analysis of SALIVA, the reader will look to that word; and for

a

account of the digestive process itself, to the article DIETETICS: in which are also several observations respecting food.

While the dissolution of the food, produced by the solvent action of the gastric juice, is going on, the two orifices of the stomach remain accurately closed. No gas ascends through the oesophagus, except when the digestive process is imperfect. Soon the muscular fibres of the organ begin to act: the circular ones, contracting at first in a vague and oscillatory manner, soon act more uniformly from above downwards, and from right to left; that is, from the œsophagus to the pylorus; while the longitudinal part approximates the two openings. The pylorus seems to possess a peculiar and exquisite sensibility, by which it distinguishes whether the substances brought in contact with it have been sufficiently acted on by the gastric juice; if that is the case, it releases and allows them to pass, while it remains closely contracted against those which are not thoroughly digested.

The time occupied by the digestive process must be expected to vary according to the constitution, age, and health of the individual, and the nature of the aliment; but it may be stated, in general, at four hours.

The action of the stomach is sometimes inverted, and the contractions, which in that case are forcible, rapid, and convul sive, cause vomiting. The exertions of

the respiratory muscles are, however, necessary to the production of this effect. Although the stomach belongs to those organs whose action is independent of the will, and goes on, therefore, without the attraction of the individual, yet it is so far influenced by the brain, that the section of its nerves entirely obstructs digestion. It sympathises most remarkably both with the constitution at large, and with particular organs.

The chyme, or semifluid substance, in⚫ to which the action of the stomach reduces the food, is propelled by the muscular power of that bag into the duodenum, where it undergoes new changes, particularly from the admixture of other animal fluids, the bile and pancreatic juice: and this process is termed chylification.

The secretion of the pancreas seems to bear a considerable analogy, both in its nature and office, to the saliva and gastric juice. It is very difficult to procure it in a pure and unmixed state from a healthy animal, but the facts which we hitherto possess shew, that its properties are very like those of saliva. It is poured into the duodenum by the contractile power of the pancreatic duct: and its secretion, like that of the salivary glands, is said to be promoted by the pressure of the stomach in its filled state; and by the stimulus of the chyme and bile on the orifice of its duct.

The bile is the fluid furnished by the liver, the largest viscus of the human body, whose importance in the animal economy is evinced by its large apparatus and complicated distribution of blood vessels, as well as by its constantly existing in all animals which have a heart.

The source of the bile has been questioned, whether it be secreted from the hepatic artery, or vena portarum? The analogy of other secretions, which are all made from arteries, favours the former opinion; but the blood of the vena portarum seems to be the fittest, from its nature, for the secretion of bile. The hepatic artery probably serves to nourish the liver, as the lungs are nourished by the bronchial artery. The distribution of the vena portarum is like that of an artery. If we suppose the hepatic artery to furnish the bile, the excretory duct would be disproportionately large.

The bile, when secreted, passes by a slow but constant stream through the hepatic duct; but a greater or less proportion is conveyed by the cystic duct into the gall-bladder, in which it is accumulated, as in a reservoir, and undergoes

certain changes of properties, which make it cystic bile. The biliary fluid probably flows directly into the intestine, while the assimilation of the food is going on; and, when the empty state of the duodenum causes the end of the ductus communis choledochus to be compressed by its muscular coat, it regurgitates into the gallbladder.

There is no direct communication between the liver and gall-bladder, and consequently no other way for the bile to enter that bag, except by the cystic duct. The hepatic bile is a thin fluid, of a clear orange colour, and slightly bitter; the cystic is viscid and tenacious, intensely bitter, and very dark coloured. The bile, thus changed by its residence in the gallbladder, is expelled by the contractile powers of the gall-bladder and cystic duct into the ductus communis, and thence passes to the duodenum. This bladder absorbs the watery parts of the hepatic bile, and adds a mucous secretion to it.

The chemical analysis, and the uses of the bile, are considered under that article.

Function of the Spleen. The situation and attachments of this organ lead us to suppose, that its uses are in some way connected with the functions of the stomach; yet there is nothing more than conjecture to be offered on this subject. The removal of the part has been performed in dogs without any material injury to the animal. Its size differs, according to the quantity of blood contained in its cells. Physiologists have stated, that its blood possesses peculiar properties; that it is more fluid, does not coagulate, nor separate readily into serum and crassamentum; is more livid, and possesses a greater quantity of carbon. As this blood goes to the liver, and as the part possesses no excretory duct, it has been argued, that its function is subservient to that of the liver, in imparting to the blood those properties which fit it for the se

cretion of bile.

By others, the spleen has been regarded as a sponge, swelling with blood when the stomach is empty, and squeezed out by the pressure of the stomach when that organ is full. Thus, it is said, more blood will go into the stomachic arteries when the secretion of the gastric juice is going on, and the superfluous part in the inactive state of the stomach will distend the spleen.

Action of the Small Intestines. The chyle, formed in the duodenum, passes through the tube of the small intestine, which is the organ for absorbing its nutritive parts.

The description of this fluid will be found under the article CHYLE. Its progress through the intestine is retarded by the numerous convolutions of the tube; and the chyle, separated from the excrementitious part of the food, is brought into contact with the inhaling orifices of the lacteals, that commence according to Lieberkuhn and other microscopical observers, by patulous orifices on the surface of the villi. The latter projections, so named from a comparison with the pill of velvet, are very numerous on the circular projecting folds of the internal coat, called valvulæ conniventes. These latter not only render the progress of the chyle slower, but increase very greatly the absorbing surface, and penetrate, in consequence of the intestinal contractions, into the midst of the chyle, in quest of its nutritive particles. A fluid is secreted from the intestine, analogous to that furnished by the stomach, although an accurate examination of its nature and properties is one of the physiological desiderata (succus intestinalis). At the same time that the absorption of the chyle from the villous surface of the intestine

is going on, it is moved gradually downwards, towards the large intestine, by the peristaltic motion. This is an undulatory and gentle constriction, taking place in several parts of the tube at once, and producing therefore a singular appearance, compared to the crawling of worms, and hence termed vermicular. It moves the chyle repeatedly over the surface of the intestine; and though it must urge that fluid partially upwards, yet its chief effect in the healthy state is exerted in the opposite direction. It is chiefly in disease that an antiperistaltic movement occurs, which conveys bile into the stomach, and even the whole contents of the small and large intestines.

By the powers now explained, which propel the alimentary mass by their contractions, and by the admixture of the various fluid menstrua which dilute and alter its properties, those memorable changes are effected, by which our food is said to be animalized or assimilated. In the duodenum and upper part of the jejunum it forms an equally mixed fluid, of the consistence of thick cream, greyish, and rather acid. Lower down it separates into two parts; the excrementitious of a pale brown or yellow colour, and nauseating smell; and the true chyle, separated from the former by the bile, and swimming on its surface.

Action of the large Intestine. The excrementitious portion of the chyme, deprived almost entirely of every nutritive portion, enters the cæcum: its passage through the last part of the ileum being favoured by a copious secretion of mucus from the glands which abound in that part. The valvula coli, which is the boundary between the large and small intestines, is designed to prevent the contents of the former from regurgitating into the latter: and it performs this office in general; for nutritive clysters would otherwise enter the small intestines, and thus enable us to administer food enough for the support of life per anum: yet it occasionally fails in its office: hence vomiting of feces, and of tobacco clysters.

The large intestine may be regarded as the organ in which the residue of the chyme undergoes its last change, viz. the conversion into feces; as a reservoir, protecting us from the disgusting necessity of having that residue constantly flowing off; and as the instrument of its final expulsion from the body.

Some absorbents exist in this last part of the alimentary tube, and convey what nutritive parts still remain in the intestinal contents; but they are comparatively few, and hence the impossibility of nourishing the body by way of clyster.

The conversion of the alimentary residue into feces is owing to some active operation of the intestine; as these are very different from the mere putrid remains of animal or vegetable matter. And when this operation is deficient, portions of the food are seen in the evacuations per anum, but slightly changed. The fecal matter is conveyed onwards by the peristaltic motion of the large intestine is becomes thicker and more consistent in its progress, and is usually formed more or less decidedly by the cells of the colon. Its natural consistence is just sufficient to retain these marks; and its appearance throughout should be homogeneous. The colour depends on the admixture of bile, and in the most natural state is of a yellow brown. Although the change which reduces the residue of our food to feces is so far from being mere putrefaction, that the excrement is in fact less prone to putrefy than other animal matter, yet there is, as in putrefaction, a disengagement of gaseous products, particularly of sulphurated hydrogen. The colouring matter of the vegetables is often seen in the feces, as the green of spinach, and red of beet-root; and the fibrous indiges

tible parts of vegetables, as the skin of fruits, husks of grains, &c. The latter, indeed, is so little affected by the powers of digestion, that when the covering is entire, a grain may pass through the body, and still retain the power to germinate.

The expulsion of the feces takes place when they arrive in the rectum, which is speedily irritated by their presence; and is performed partly by the muscular coat of that intestine, and partly by the muscles of respiration, producing the effort called straining. The periods of voiding the excrement vary, from several causes: they are more frequent in the young subjects, where the stools are more liquid. In the adult they should not be less, in a healthy state, than once in twenty-four hours.

Urinary Secretion. The liquids which we drink, absorbed by the lacteals, together with the nutritive part of the solid aliment, dilute the latter, and serve as a vehicle for it. They increase the quantity of the blood, and render it more fluid; conveyed into every part of the circulating system, they penetrate all our organs, carry away the particles' detached from them in the different vital processes, and are then separated from the mass of fluids by the urinary organs, together with various other substances, whose retention in the body would produce injurious effects. The kidneys, therefore, dispose of the residual part of our liquid ingestu, as the feces are formed by the more solid foods, and the quantity of urine may, of course, be expected to vary according to that of the drink. All the old parts of the frame, which are constantly removed by the absorbents, while new depositions are formed by the nutrient arteries, go off in the same way; and hence the urine, although apparently a watery fluid, and called in common language water, contains a great deal of

animal matter.

From the above account it will be readily understood, that the properties of the urine must vary according to the time at which it is voided after meals, the quantity of food and drink, the age, and complaints of individuals, &c. Physiologists have distinguished urine of the drink, chyle and blood. The first is a watery fluid, almost colourless, evacuated very soon after drinking, and possessing very slight urinous characters; the second, evacuated two or three hours after meals, is better elaborated, but not yet complete in its constituent principles; the last,

voided after the repose of the night, has all the properties of urine in an eminent degree. In infants it possesses no phosphate of lime nor phosphoric acid, as those substances are employed in the business of ossification, which is then active. In old persons, on the contrary, where the bony system, already overcharged with phosphate of lime, refuses to admit any more, this substance is carried off by the kidneys. It is removed in the same way in rachitis and mollities ossium, where the bones become softened by its absence.

The great quantity of saline and crystallizable elements contained in the urine account for the frequency of calculous concretions, which are found by recent and accurate analysis to vary very considerably in their composition. As there is no substance in the body which may not be evacuated by the urine, and manifest itself in that liquid, so, under different circumstances, every thing possessing a power of concretion may become the subject of urinary calculus. This diversity of constituent elements, together with the want of characteristic symptoms of the different species, and the irritation which the coats of the bladder must experience from chemical reagents, will convince us how extremely difficult, if not impossible, it must be, to discover a lithontriptic that would obviate the necessity of a surgical operation.

The urine is very speedily and sensibly affected by certain substances; thus asparagus occasions a remarkable fœtor in this fluid; and turpentine imparts to it a violet odour. For a further account of its composition and physiology, see the article URINE.

Almost every physiologist has noticed the rapidity with which this secretion is carried on aqueous fluids, taken by the mouth, are sometimes separated so quickly by the kidneys, that an immediate communication has been suspected between the stomach and kidneys, on the supposition that there had not been a sufficient time for the fluid to arrive at the latter organ in the regular course of absorption and circulation. This conjecture derives no countenance from anatomy, and the size of the renal vessels explains the fact without any such supposition.

Absorption, or the process by which the chyle, separated from the food by the digestive organs, is carried into the blood, naturally follows the account of diges tion. We have very little to add to what