479. The composition of the Bile, and the structure of the organ which elaborates it, will be more appropriately considered hereafter, when the Secreting apparatus generally is being described (CHAP. IX.) At present we have to inquire what is the precise effect of its admixture with the products of digestion, and what is the purpose which this admixture serves. In the first place, it may be stated that biliary matter is essentially a soap, formed by the union of a fatty acid with a sodabase; and that it serves the purpose of neutralizing the acidity of the chyme, which is derived from the gastric juice; the biliary acids falling down as an insoluble precipitate, when thus deprived of their soda. Further, the bile shares with the pancreatic fluid in that emulsifying power, by which the fatty matters of the food are reduced to a state of such fine division, as to be rendered capable of being absorbed; and thus it happens that the introduction of these matters into the system, through the medium of the lacteal absorbents (§ 494), does not take place until after the chyme has been mingled with the biliary and pancreatic secretions. Again, it has been asserted (but the fact has not been fully substantiated), that the admixture of biliary matter produces a conversion of saccharine in fatty compounds. When fresh bile is mingled with newly-formed chyme, in a glass vessel, the mixture separates into three distinct parts; a reddish-brown sediment at the bottom, a whey-colored fluid in the centre, and a creamy pellicle at the top. The central stratum probably contains the albuminous, gelatinous, saccharine, and other matters in a state of solution; the superficial pellicle may be looked upon as consisting chiefly of oleaginous matter destined for absorption; whilst the sediment, partly consisting of the unreducible portion of the food, and partly of the biliary matter itself, is evidently excrementitious.

480. The Pancreatic secretion has a chemical constitution very analogous to that of Saliva; but the peculiar organic compound which it contains, has been found by M. Bernard to possess a power of emulsify⚫ing fatty matter, when mingled with it; and there is strong reason to believe that the chief purpose of this secretion is to effect such a change in the condition of the oleaginous constituents of the chyme, as may prepare them for absorption. But further, the neutralization of the acid of the gastric fluid now allows the metamorphosis of starch to be recommenced; and as there is evidence that the production of sugar continues to take place during the passage of the chymous mass along the small intestines, in animals whose food is partly or completely vegetable, the pancreatic fluid, which has been experimentally ascertained to possess this power, is probably the chief agent by which the conversion is effected. It may be surmised, further, that the glandulæ of Brunner (§ 450) participate in the functions of the Pancreas; being, perhaps, the chief agents in the elimination of that "succus entericus," which has been. experimentally found to concur with the biliary and pancreatic fluids in the emulsification of fatty matters.

481. During the passage of the contents of the Intestine, now augmented by the biliary secretion, along the canal, the nutritious portion is gradually withdrawn by the absorbent vessels on its walls; and the excrementitious matter alone remains, increased in amount by the pro

ducts of the secretion of the Peyerian and other glandulæ, with which the mucous lining of the lower intestines is studded. Many of the lower animals are furnished, at the part where the small intestine enters the large, with a cæcum, resembling that which in Man is termed the vermiform appendage of the cæcum, but greatly exceeding it in size. Sometimes we find two cæca instead of one; and these are much prolonged, so as to form tubes of considerable length. It has been ascertained that, in herbivorous animals, a distinctly acid secretion is formed by the cæcum, during the digestive process; and there is reason to believe, that the food there undergoes a second process, analogous to that to which it has been submitted in the stomach, and fitted to extract from it any undissolved alimentary matter which it may still contain. There is no reason to believe, however, that any such process takes place in Man, whose real cæcum is rudimentary,-the part of the intestine which has received the name, being merely the dilated commencement of the colon. The act of Defecation, by which the excrementitious matter is discharged, has been already noticed (§ 462); the Absorption of nutritive matter will be treated of in the succeeding Chapter.

5. Of Hunger, Satiety, and Thirst.

482. The want of solid aliment is indicated by the sensation of Hunger; and the deficiency of fluid by that of Thirst. On the other hand, the presence of a sufficiency of food or liquid in the stomach is indicated by the sense of Satiety. These sensations are intended as our guides, in regard to the amount of aliment we take in. What is the real seat of these sensations, and on what conditions do they depend?

483. The sense of Hunger is referred to the stomach, and seems immediately to depend upon a certain condition of that organ; but what that condition is, has not yet been precisely ascertained. It is not produced by mere emptiness of the stomach, as some have supposed; for, if the previous meal have been sufficient, the food passes entirely from the cavity of the stomach, before a renewal of the sensation is felt. It cannot be due to the action of the gastric fluid upon the coats of the stomach themselves; because this fluid is not poured into the stomach, except when the production of it is stimulated by the irritation of the secreting follicles. It has been attributed to distension of the gastric follicles by the secreted fluid; but there is no evidence that the fluid is secreted before it is wanted; and moreover, it is well known that mental emotion can dissipate in a moment the keenest appetite, and it is diffi cult to imagine how this can occasion the emptying of the follicles. Perhaps the most satisfactory view is that, which attributes the sense of hunger to a determination of blood to the stomach, preparing it for the secretion of gastric fluid; since this is quite adequate to account for the impression made upon the nerves; and it accords with what has just been stated of the influence of mental emotions, since we know that these have a powerful effect upon the circulation of blood in the minute vessels (§ 603).

484. Although the sense of Hunger is immediately dependent, in

great part at least, upon the condition of the stomach, yet it is also indicative of the condition of the general system; being extremely strong, when the body has undergone an unusual waste without a due supply of food, even though the stomach be in a state of distension; whilst it is not experienced, if, through the general inactivity of the system, the last supply has not been exhausted, even though the stomach has been long empty. It is well known that, when food is deficient, the attempt to allay the pangs of hunger by filling the stomach with non-nutritious substances, is only temporarily successful; the feeling soon returning with increased violence, though it has received a temporary check. The reason for this is obviously, that the general system has received no satisfaction, although the stomach has been caused to secrete gastric fluid by the contact of solid matter with its walls: so that although the state on which hunger immediately depends, has been for a time relieved, this state is soon renewed, unless the solid matter introduced into the stomach be of an alimentary character, and be dissolved and carried into the system.

485. When the food is nutritious in its character, but of small bulk experience has shown the advantage of mixing it with non-nutritious substances, in order to give it bulk and solidity; for if this be not done, it does not exert its due stimulating influence upon the stomach; the gastric juice is not poured forth in proper quantity; and the result is, that neither is the sense of hunger relieved, nor are the wants of the body satisfied. Thus the Kamschatdales are in the habit of mixing earth or sawdust with the train-oil, on which alone they are frequently reduced to live. The Veddahs or wild hunters of Ceylon, on the same principle, mingle the pounded fibres of soft and decayed wood with the honey on which they feed when meat is not to be had; and on one of them being asked the reason of the practice, he replied, "I cannot tell you, but I know that the belly must be filled." It has been found that soups and fluid diet are not more readily converted into chyme than solid aliment, and are not alone fit for the support of the body in health; and it is often to be observed, in disordered states of the stomach, that it can retain a small quantity of easily digested solid food, when a thin broth would be rejected.

486. The sense of Satiety is the opposite of Hunger; and like it, depends on two sets of conditions, the state of the stomach, and that of the general system. It is produced in the first instance by the ingestion of solid matter into the stomach, which gives rise to the feeling of fulness; but this is only a part of the sensation which ought to be experienced; and it is only when the act of digestion is being duly performed, and nutritive matter is being absorbed into the vessels, that the peculiar feeling of satisfaction is excited, which indicates that the wants of the system at large are being supplied.-It has been very justly remarked by Dr. Beaumont, that the cessation of the demand set up by the system, rather than the positive feeling of satiety, should be the guide in regulating the quantity of food taken into the stomach. The sense of satiety is beyond the point of healthful indulgence; and is Nature's earliest indication of an abuse and overburden of her powers

to replenish the system. The proper intimation is the pleasurable sensation which is experienced, when the cravings of the appetite are first allayed; since, if the stomach be sufficiently distended with wholesome food for this to be the case, it is next to certain that the digestion of that food will supply what is required for the nutrition of the body. It is only when the substance with which the stomach is distended, is not of a digestible character, that the feelings excited by the state of that organ are anything but a correct index of the wants of the system.

487. The Par Vagum is evidently the nerve, which conveys to the sensorium the impression of the state of the stomach, and which is therefore the immediate excitor of the sensation of hunger or of the feeling of satiety. But it is evident from experiments upon animals, that it is not the only source, through which they are incited to take food, and are informed when they have ingested enough; and it is probable that the Sympathetic nerve is the channel, through which the wants of the system are made known, and through which, in particular, the feeling of general exhaustion is excited, that is experienced when there has been an unusual waste, or when the proper supply has been too long withheld.

488. The conditions of the sense of Thirst are very analogous to those of hunger; that is, it indicates the deficiency of fluid in the body at large; but the immediate seat of the feeling is a part of the alimentary canal,-not the stomach, however, but the fauces. It is relieved by the introduction of fluid into the circulating system, through any channel; whilst the mere contact of fluids with the surface to which the sensation is referred, produces only a temporary effect, unless absorption takes place. If liquids be introduced into the stomach by an œsophagus-tube, they are just as effectual in allaying thirst, as if they were swallowed in the ordinary manner; and the same result follows the injection of fluid into the veins (as was most remarkably the case when this method of treatment was practised in the Asiatic Cholera), or the absorption of fluid through the skin or the lower part of the alimentary canal. The deficiency of fluid in the body may arise, and Thirst may consequently be induced,-either by an unusually small supply of fluid, or by excessive loss of the fluids of the body, as by perspiration, diarrhoea, &c. But it may also be occasioned by the impression made by particular kinds of food or drink upon the alimentary canal; thus salted or highly-spiced meat, fermented liquors when too little diluted, and other similar irritating agents, excite thirst; the purpose of which sensation is evidently to cause the ingestion of fluid, by which these substances may be diluted, and their irritating action prevented.

CHAPTER V.

ABSORPTION AND SANGUIFICATION.

1. Absorption from the Digestive Cavity.

489. So long as the Alimentary matter is contained in the digestive cavity, it is as far from being conducive to the nutrition of the system, as if it were in contact with the external surface. It is only when absorbed into the vessels, and carried by the circulating current into the remote portions of the body, that it really becomes useful in maintaining the vigor of the system, by replacing that which has decayed, and by affording the materials for the various organic processes which are continually going on. Among the Invertebrated animals, we find the reception of alimentary matter into the circulating system to be entirely accomplished through the medium of the veins, which are distributed upon the walls of the digestive cavity. We not unfrequently observe, that the intestinal tube is completely enclosed within a large venous sinus, so that its whole external surface is bathed with blood; and into this sinus, the alimentary materials would appear to transude, through the walls of the intestinal canal, to become mingled with the blood, and to be conveyed with its current into the remote portions of the body. Among the Vertebrata, we find an additional set of vessels, interposed between the walls of the intestine and the sanguiferous system, for the purpose, as it would seem, of taking up that portion of the nutritive matter which is not in a state of perfect solution, and of preparing it for being introduced into the current of the blood. These vessels are the lacteals or absorbents. They are very copiously distributed upon the walls of the smaller intestine, commencing near the entrance of the biliary and pancreatic ducts; the walls of the large intestine are less abundantly supplied with them, and they do not show themselves in the villi which are found on some parts of the lining membrane of the stomach, although the walls of that viscus are supplied with lymphatic absorbents.

490. Nevertheless it is quite certain, that substances may pass into the current of the circulation, which have been prevented from passing further than the stomach; thus, if a solution of Epsom-salts be introduced into the stomach of an animal, and its passage into the intestine be prevented by a ligature around the pylorus, its purgative action will be exerted nearly as soon, as if the communication between the stomach and intestines had been left quite free; or if a solution of prussiate of potash be introduced into the stomach under similar circumstances, the presence of that salt in the blood may be speedily demonstrated by chemical tests. It appears from the experiments of MM. Tiedemann and Gmelin, that when various substances were mingled with the food, which, by their color, odor, or chemical properties might be easily detected,-such as gamboge, madder, rhubarb, camphor,