LACTEAL SYSTEM. THORACIC DUCT. 797 



deep fissures into several lobes, a disposition whereby the free movement 

 of this part of the body is evidently facilitated. The gall-bladder, when 

 present, which is not invariably the case, receives the bile indirectly 

 through a cystic duct derived from the hepatic ; so that the biliary fluid, 

 poured into the duodenum through a ductus communis choledochus, is 

 derived either immediately from the liver, or is regurgitated from the 

 gall-bladder as occasion requires. 



(2322.) The pancreas resembles the human in every particular; and 

 its secretion enters the duodenum at the same point as that of the 

 liver. 



(2323.) The spleen is always attached to the stomach by a dupli- 

 cature of the peritoneal lining of the abdomen, and is organized in the 

 same manner as that of Man, except in the CETACEA, where this viscus 

 is divided into several small portions quite distinct from each other. 



(2324.) The system of the vena portce is made up of the venous 

 trunks derived from the spleen, the stomach, the pancreas, and the in- 

 testinal canal : these all unite to form one large central trunk, which, 

 after entering the liver, again divides and subdivides minutely in that 

 viscus, and furnishes the venous blood, from which the bile is principally, 

 if not entirely, elaborated. 



(2325.) The peritoneum, or the serous membrane lining the abdomi- 

 nal cavity, forms in the Mammalia a shut sac, and by its numerous in- 

 flexions invests all the chylopoietic viscera, forming broad mesenteric 

 folds to support the intestines ; it thus encloses between its laminae the 

 entire system of mesenteric vessels, and also the lacteals derived from 

 the alimentary canal : as to the rest, its structure and disposition, even 

 to the formation of the omental sacs, differ in no important respect from 

 what is found in the human body. 



(2326.) The chyle, the result of the digestive process, is taken up 

 from the mucous lining of the intestinal canal by innumerable micro- 

 scopic orifices that form the commencement of the lacteal system, which 

 in the Mammalia seems to assume its most perfect development. This 

 important system of absorbent vessels consists of slender canals enclosed 

 between the two layers of the mesentery, to the root of which they con- 

 verge from all the tract of the intestine. The valves formed by the 

 lining membrane of these tubes are in Mammals so numerous and perfect 

 that it is no longer possible to inject them from trunk to branch. Before 

 terminating in the thoracic duct, these vessels permeate numerous 

 " mesenteric glands," as they are called, by means whereof they appear 

 to communicate freely with the venous system ; but the bulk of the 

 matter absorbed enters a kind of reservoir called the " receptaculum 

 chyli" whence, by means of the thoracic duct, the chyle is conveyed to 

 be mixed up with the mass of circulating fluid, and is ultimately poured 

 into the vena innominata at the junction of the jugular and subclavian 

 veins of the left side of the body. 



