THE LACTEALS AND THE LYMPHATIC SYSTEM. 313 



carried out by means of microorganisms, marsh gas being one of the prod- 

 ucts formed at the same time. 



Be this as it may, whether digestion^ properly so called, is all but com- 

 plete at the ileo-ca3cal valve, or whether important changes still await 

 the chyme in the large intestine, one great characteristic of the work 

 done in the colon is absorption. By the abstraction of all the soluble con- 

 stituents, and especially by the withdrawal of water, the liquid chyme 

 becomes as it approaches the rectum converted into the firm solid feces, and 

 the color shifts from the bright orange, which the gray chyme gradually 

 assumes after admixture with bile, into a darker and dirtier brown. 



TJie Feees. 



245. These consist in the first place of the indigestible and undi- 

 gested constituents of the meal ; shreds of elastic tissue, hairs and other 

 horny elements, much cellulose and chlorophyll from vegetable, and some 

 connective tissue from animal food, fragments of disintegrated muscular 

 fibre, fat-cells, and not unfrequently undigested starch-corpuscles. The 

 amount of each must of course vary very largely according to the nature 

 of the food, and the digestive powers, temporary or permanent, of the indi- 

 vidual. In the second place, to these must be added substances not dis- 

 tinctly recognizable as parts of the food, but derived from the secretions of 

 the alimentary canal. The feces contain mucus in variable amount, some- 

 times albumin, cholesterin, butyric and other fatty acids, lime and magnesia 

 soaps, coloring matters, and inorganic salts, especially earthy phosphates, 

 crystals of ammonio-magnesia phosphates being very conspicuous. The re- 

 action is generally but not always acid. They also contain a ferment similar 

 in its action to pepsin, and an amylolytic ferment similar to that of saliva 

 or of pancreatic juice. The bile salts are represented by a small quantity 

 of cholalic acid, or some product of that body, and sometimes a very small 

 quantity of taurin. The glycin and most or all of the taurin have been ab- 

 sorbed from the intestine, and the cholalic acid has been partly absorbed 

 and partly decomposed. The fact that the feces become " clay-colored " 

 when the bile is cut off from the intestine shows that the bile-pigment is at 

 least the mother of the fecal pigment ; and a special pigment, which has been 

 isolated and called stercobilin, is said to be identical with the substance called 

 urobilin, which may be formed from bilirubin. As other special constitu- 

 ents of the feces may be mentioned excretin, a somewhat complex nitrogen- 

 ous body, whose exact chemical nature is at present uncertain, and sfcatol 

 (C 9 H 9 N), a nitrogenous body which like indol is derived from the decompo- 

 sition of proteids by means of microorganisms, and which is the chief cause 

 of the fecal odor, since only a small quantity of indol remains in the feces. 

 These odoriferous bodies are derived directly from the food ; at the same 

 time it is quite possible that other specific odoriferous substances may be 

 secreted directly from the intestinal wall, especially from that of the large 

 intestine. 



THE LACTEALS AND THE LYMPHATIC SYSTEM. 



246. We have seen that absorption does, or at least may, take place 

 from the stomach. We have also stated that a large absorption, especially 

 of water, occurs along the whole large intestine. Nevertheless, it is during 

 the transit of food along the small intestine that the largest and most im- 

 portant part of the digested material passes away from the canal, partly into 

 the lacteals, partly into the portal vessels. The portal vessels are simply 

 parts of the general vascular system ; the lacteals, into which, we may at 



