We shall conclude this part of the subject with a few remarks on the chemical com position of the contents of the small intestine. On laying open the gut, we usually find a semi-solid admixture of imperfectly digested and indigestible substances and of the constituents of the digestive fluids in a more or less changed condition. The reac tion of this mass varies in different parts of the canal, and in some measure with the nature of the food. • Thus, the contents of the stomach always redden litmus paper, whatever kind of food has been taken; the duodenal contents are also always acid, but in a far less intense degree; in the jejunum we meet with only a faint acid reaction, which altogether disappears in the ileum; while in the emcum, and sometimes in the lower part of the ileum, an alkaline reaction occurs. After a purely flesh diet the acid reaction disappears shortly below the duodenum, while, after the sole use of vegetable food, it may sometimes be traced even to the cfecum. As a general rule, the con tents of the large intestine are alkaline.
In consequence of the rapid absorption that goes on along the intestinal surface, we meet with a comparatively small amount of soluble matters in these contents. Among these soluble matters we often find glycose (or grape-sugar), which seems to owe its origin to the metamorphosis of starch, and not to sugar having been present in the food; for after saccharine food has been taken, we rarely meet with it in any quantity in the small intestine, its absorp tion taking place with great rapidity. In the alcoholic extract of these contents we can almost always find evi dence of the presence of biliary constituents. In the duo denum, and for a little way beyond it, we find glyco cholic and tauro-cholic acid; descending a little further, they rapidly diminish, till we find the products of their disintegration; while in the large intestine, little more than a trace of these products can be detected. These chemical observations confirm the experiments of Schmidt, which show that nearly half the bile which is poured into the duodenum is decomposed before it reaches the middle of the small intestine.
We have now arrived at the seventh stage of the diges tive process, that of defecation. The line of demarcation between the small and large intestine is very obvious, and by the peculiar arrangement of the ileo-emcal valve (see fig.
10), matters are allowed to pass forward with facility, while regurgitation is impossible. For anatomical details regarding the large intestine, we may refer to the articles ALIMENTARY CANAL, Ciectnit, and COLON. The contents of the large intestine differ very materially from those which we have been considering in the last paragraph, and constitute the fwees. They are more solid and homogeneous. and are often molded into a definite shape by the cells of the colon. The only essential change which the contents undergo in this part of their course is. that they increase as they pass onward in solidity, in consequence of the absorption of fluid from then'by the mucous membrane.' They are propelled forward into the rectum by the vermicular action which nas been already described. Here they accumulate, being prevented from escaping by the contraction of the sphincter muscle—a band of strong muscular fibers surrounding and closing the gut at its lower extremity. The act of defecation, or of expulsion of the farces from the rectum, is effected partly by the muscular fibers of that part of the intestine which are stimu lated to contraction by a certain degree of distension, and which are to a certain extent under the influence of the will, and partly by the simultaneous contractions of the abdominal muscles and of the diaphragm, which, by reducing the antero-posterior and transverse diameters of the abdominal cavity, compress the intestinal canal in such a manner as greatly to assist the expulsive action of the rectum. These forces, or some of them (for usually the detrusive action of the muscular fibers of the rectum is suffi cient), overcome the passive contraction of the sphincter, and the act of defecation is the result.
The fteces consist of a mixture composed of undigested particles of food (such as vegetable cellular tissue, fragments of tendon, skin, and half-digested muscular fiber), of epithelium and mucus (derived from the intestinal walls), and of traces of decomposed biliary matters. Their peculiar odor is ascribed by some to the secretion of Foyer's glands, and by others to decomposed bile; while Liebig refers it to a decomposition of albuminous matters, founding his view upon the fact, that by burning albumen with potash, he could manufacture in the laboratory odors of a fecal character. The last is the least probable view. Their color varies with the food. On a mixed diet, they are of a yellowish-brown tint; on a flesh diet, much darker; and on milk diet, quite yellow —and they become darker on exposure to the air. Their reaction is most commonly but not invariably alkaline. Their daily quantity is very variable; the mean of 17 observations made by a German physiologist, Wehsarg, was about 4.6 ounces, of which very nearly one ounce was solid matter, the rest being water; the largest and the smallest quantities being ten, and rather more than two ounces. Liebig, many years ago, made the observation that the insoluble salts of the food are mainly carried off by the freces, while the soluble salts are for the most part eliminated by the urine. For further details on the chemistry of this subject, we must refer to the elaborate Memoirs of Dr. Marcet, published in the Fhilosophical Transactions, 8. The absorption of the chyle forms the completion of the digestive act. The coats of the intestines contain two perfectly distinct sets of vessels—one through which blood circulates, and the other containing a milky or transparent fluid, chyle or lymph, which, after a somewhat circuitous route, is poured into the blood. We have already rererred to the fact, that fluids are absorbed from the stomach and intestine by the veins and capillaries of the mucous membrane; we now proceed to notice the mode in which the vessels of the second kind, the lacteals, act as absorbing agents. The lacteals are merely a portion of the great lymphatic system of the body, which will be described in a future article. See LYMPHATICS. They commence, as has been previously mentioned, in the villi, and possibly also in the intervening mucous membrane; and when an animal is killed while the digestive process is going on, they have, in consequence of their being distended with chyle, the peculiar white or milky appearance which procured for them their name of rasa lactea, from their discoverer, Asellius, in 1622. They pass in great numbers, and in a reticulated arrangement, between the layers of the mesentery, the portion of peritoneum (q.v.) which surrounds the gut, and retains it in its proper position. After passing through the mesenteric glands, where their contents seem to become more highly organized, they make their way to the right side of the aorta in the lumbar region, where they finally discharge themselves into an elongated pouch, termed the receptaculum &fill. From this pouch, the thoracic duct, containing the chyle, passes upward along the vertebral column till it reaches the level of the arch of the aorta, behind which it runs to the left side, and discharges its contents into the subclavian vein, close to its origin with the internal jugular, its orifice being protected by two valves. The nature of these contents has been already described in the article CHYLE. This chyle is, in reality, incipient blood, which has been formed, as we have already seen, from the food, and has been absorbed from the intestine by the lacteals. We have now traced it to its entrance into the general circulation, and it only remains for it to pass, in conjunction with the venous blood with which it is mixed, through the lungs, in order to be converted into new and perfect arterial blood, fit for the highest processes forranization.