682 MAMMALIA. 



(774.) The system of the vena port<p is made up of the ve- 

 nous trunks derived from the spleen, the stomach, the pancreas, 

 and the intestinal canal : these all unite to form one large cen- 

 tral trunk, which after entering the liver again divides and sub- 

 divides minutely in that viscus, and furnishes the venous blood, 

 from which the bile is principally if not entirely elaborated. 



(775.) The peritonaeum, or the serous membrane lining the 

 abdominal cavity, forms in the Mammalia a shut sac, and by its 

 numerous inflexions 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 hu- 

 man body. 



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

 up from the mucous lining of the intestinal canal by innumerable 

 microscopic orifices that form the commencement of the lacteal 

 system, which in the Mammalia seems to assume its most perfect 

 developement. This important system of absorbent vessels con- 

 sists of slender canals enclosed between the two layers of the me- 

 sentery, to the root of which they converge 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 ter- 

 minating in the thoracic duct, these vessels permeate numerous 

 4t 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 the 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. 



(777.) The lymphatic system of Mammals, as far as it has 

 been studied, conforms in its arrangement to that of Man. 



(778.) Neither will it be at all necessary to describe at any 

 length the construction of the respiratory and circulatory organs 

 in the class now under consideration ; seeing that the structure of 

 the lungs, the mechanism of respiration, the arrangement of the 

 pulmonary vessels, the cavities of the heart, and the general dis- 



