840 MAMMALIA. 



and in the Human female, it forms a single vascular cake, whence is 

 derived the name appropriated by anatomists to this important viscus. 



(2490.) After the development of the placental system, it is obvious 

 that the arteries derived from the common iliac trunks of the foetus, 

 which at first were distributed only to the allantois, as in the case of 

 the Bird ( 2139), on the development of the placenta become trans- 

 ferred to the latter viscus, and form the umbilical arteries of the navel- 

 string. The vein, likewise, notwithstanding its prodigiously-increased ex- 

 tent of origin after the placenta has been formed, takes the same course 

 on entering the umbilicus of the foetus as it did when it was derived 

 only from the allantois ; so that, although the placenta completely usurps 

 the place of the allantois, both the allantoic and placental circulations 

 are carried on through the same umbilical arteries and veins. 



(2491.) In order to complete our history of foetal development up to 

 the full establishment of the permanent double circulation that charac- 

 terizes all the hot-blooded Vertebrata after birth, it only remains for 

 us to notice the changes that occur in the vessels of the foetus, whereby, 

 on the cessation of the functions of the placenta, the pulmonary circu- 

 lation is at length brought into action. 



(2492.) Up to the period of birth the arrangement of the foetal cir- 

 culation remains essentially that of a Eeptile, inasmuch as both the 

 venous blood derived from the system and the arterialized blood that 

 comes from the placenta are mixed together in the as yet imperfectly 

 separated chambers of the heart. Under these circumstances the 

 arrangement of the vascular system is as follows : Pure blood, supplied 

 from the placenta, is brought into the body by the umbilical vein, which 

 passes partly into the portal system of the liver, but principally through 

 the ductus venosus into the inferior cava, and thence into the heart. 

 From the construction of the heart during this portion of foetal exist- 

 ence it is obvious that in that viscus all the blood derived from the 

 placenta, from the venous system of the foetus, and also from the as yet 

 inactive lungs, is mingled together prior to its distribution through the 

 arterial system. The two auricles communicate freely with each other 

 through the foramen ovale ; and, by means of the ductus arteriosus, the 

 greater portion of the blood driven from the right ventricle during the 

 systole of that cavity passes into the aorta, a veiy small proportion 

 only finding its way into the pulmonary arteries. Such a heart, there- 

 fore, supplies to the foetal system a mixed fluid, of which a portion, 

 having passed through the arterial trunks, finds its way back to the 

 placenta through the two umbilical arteries, there to recommence the 

 same circle. 



(2493.) Immediately after birth, however, the whole arrangement is 

 altered, and the adult condition fully established. The lungs assume 

 their functions and the pulmonary arteries attain their full proportions, 

 while the placenta at once ceases from its office, and all the umbilical 



