NVe shall conclude this part of the subject v, ill a few remarks on the chemical composition of the contents of the small intestine. ()II laying open the gut we usually timid a semi-solid ad mixture of imp•rfe•tly digested and indigestible substances, and of the constituents of the diges tive fluids in a more or less, changed condition. The reaction of this mass varies in ditTerent parts of the canal, and in with the na ture of the food. Thus. the VOW elltthe Stt)111• :It'll a INN :IS=. redden Inns. tI1111.; paper, whatever kind of has been taken; the duodenal contents are also always acid, lint in a far less intense degree; in the we meet with only it faint acid re action. which altogether disappears in the ileum: while in t :11141 in the hitter part of the- ileum, an alkaline reaction occurs. .totes a purely animal diet the ,tteid reaction dis appears shortly helm the duodenum: while. after the sole use of vegetable food, it mat' sometimes be traced even to the ewennt. As a general rule the eontents 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 itt these contents. Among soluble matters we often find glucose (or grape-sugar), which seems to incits origin to the metamor phosis of stareh. and not to sugar having been present in t he food: for after charine food has 'Well taken we rarely meet with it in any quan tity in the small in testine. its allsorption taking place great rapidity. In the alcoholic extract of these 11 can almost find evidence of the presence of bill:try constituents. In the dmidenum, aunt 1.a. a little way beyond it we find gi,cocholic t it• acid: de scending a little far ther they rapidly dim inish. till we find the products of their dis integral ; while in the1:I more than a trace of these prod ucts can be deleeted. 4111'111Iva] (Ibsen 0• lions conlitin the experiments of !-:•einnidt, ithich show that nearly half the bile which is poured into the duodenum is decomposed before it reaches the middle of the small intestine.
(71 \V• have now arrived at the seventh stage of t he digestive proce•o:--111:It of ration. The line of d•m:ire:Ohm 1 I .)e.‘%evit the large intestine is very oh\ ions, and by the peculiar ar rangement of the ileo•teetil valve (see rig. 10) matters are allowed to pass forward with faeil ity. while regurgitation is impossible. ( For details regarding the large intestine. we may refer to t he articles Am NIENTAny tSysTE3t; Caveat: 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 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 them by the mu cous membrane. They are propelled forward into the rectum by the vermicular action which has been already described. 'Here they accumulate. being prevented from escaping by the contraction of the sphincter nm:cle—a band of strong muscu lar fibres surrounding and closing the gut at its lower extremity. The act of defecation, or of ex
pulsion of the faeces from the rectum. is effected partly by the muscular fibres of that part of the intestine which are stimulated to contraction by a certain degree of distention, 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 usu ally the detrnsive action of the muscular fibres of the rectum is sufficient). overcome the passive contraction of the sphincter. and the act of def ecation is the result. See F.ECES.
(S) The absorption of the chyle forms the com pletion of the digestive act.,, The coats of the intestines contain two perfectly distinet 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 referred 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 absorhing agents. The laeteals are merely a por tion of the great lymphatic system of the body. which will he described in a future article. (See Lvmmt.mcs.) They commenee, as has been previously mentioned, in the 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 tarter:. from their dis coverer. Asellins. in 1622. They pass in great numbers, and in a retienlated arrangement. be tween 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, %•iteno their con tents seem to become more highly organized. they make their way to the right side of the aorta in the lumbar region. where they finally dis charge themselves an elongated pouch, termed the rereptaeulum rhyli. From this pouch the thoracic duet, •ontaining the chyle. passes upward along the vertebral column till it reaches the level of the arch of the aorta, be hind which it runs to the left side and discharges its contents into the subelavian 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 of organization.