SANGUIFEBOUS SYSTEM. 51/ 



capillaries of these venous and arterial ramifications, through 

 the mass and around the tubuli biliferi of the liver, constitute 

 the roots of the hepatic veins (142. A. 8. 9.), which enter the 

 inferior cava before it perforates the diaphragm and pericar- 

 dium to open into the lower part of the right auricle. 



From the right ventricle, the entire venous blood of the 

 system is sent through a single pulmonary artery (142. A. 

 /.), provided with three semilunar valves with thickened mar- 

 gins, with corpuscula in their middle, and with fossae be- 

 hind them. The pulmonary artery divides into a right and 

 left trunk (142. A. n. m.), each of which subdivides accord- 

 ing to the divisions of the bronchi which they accompany, 

 and to the number of the lobes of the lungs, around the 

 cells of which they are distributed, as around the tubuli 

 and cells of a gland the lungs being merely a gland for 

 secreting carbon, originating with a blastema and deve- 

 loping as other glands, and sending its secretion in a gaseous 

 form through its duct, the trachea. The blood arterialized 

 in the capillaries of the lungs, is returned to the left side of 

 the heart, by a variable number of pulmonary veins, which 

 open commonly by four or six or sometimes by two orifices 

 unprovided with salves, or even by a single aperture, into 

 the left auricle. 



There is thus great unity of plan in the distribution of the 

 arteries and veins throughout the vertebrated classes, and 

 there is much analogy between the vessels of the anterior and 

 the posterior portions of the trunk, and between those of the 

 atlantal and sacral extremities. The common iliac arteries 

 and veins are analogous to the arterial and venous brachio- 

 cephalics, and their branches mutually resemble. The 

 superior vena cava is single in the higher mammalia and in 

 man, because their heart is placed farther backwards from 

 the head, and the brachio-cephalic veins are thus enabled to 

 unite before they arrive at that cavity. But in the inferior 

 tribes, where the heart is situate more anteriorly in the trunk, 

 the two brachio- cephalics enter that organ as separate superior 

 venae cavfc, before the extent of their course has allowed them 

 to meet and unite. The posterior vena cava is longer than 

 the anterior, throughout the vertebrata, as the abdominal or 

 posterior portion of the aorta is longer than the brachio- 

 cephalic or anterior trunks, because the heart, in this highest 

 subkingdom of animals, constantly retreating from the head 



