730 MAMMALIA. 



(860.) The appearance of the placenta varies much in different 

 tribes : thus, in the Sheep and other RUMINANTS it consists 

 of numerous detached masses of villi (z, z), that indigitate with 

 corresponding processes derived from the maternal womb ; in 

 the Mare it covers the whole surface of the chorion ; but in the 

 greater numbers of Mammals, and in the Human female, it forms a 

 single vascular cake, whence is derived the name appropriated by 

 anatomists to this important viscus. 



(861.) After the developement of the placental system, it is 

 obvious that the arteries derived from the common iliac trunks of 

 the fetus, which at first were distributed only to the allantois, as 

 in the case of the Bird ( 705), on the developement of the pla- 

 centa become transferred to the latter viscus, and form the umbili- 

 cal arteries of the navel-string. The vein likewise, notwithstand- 

 ing its prodigiously increased extent of origin after the placenta 

 has been formed, takes the same course on entering the umbilicus 

 of the fetus 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. 



(862.) In order to complete our history of fetal developement 

 up to the full establishment of the permanent double circulation 

 that characterises all the hot-blooded Vertebrata after birth, it 

 only remains for us to notice the changes that occur in the vessels 

 of the fetus, whereby, on the cessation of the functions of the pla- 

 centa, the pulmonary circulation is at length brought into action. 



Up to the period of birth the arrangement of the fetal circula- 

 tion remains essentially that of a Reptile, 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 im- 

 perfectly separated chambers of the heart. Under these circum- 

 stances 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 fetal existence it is obvious, that, in 

 that viscus, all the blood derived from the placenta, from the ve- 

 nous system of the fetus, and also from the as yet inactive lungs, is 

 mingled together prior to its distribution through the arterial sys- 

 tem. The two auricles communicate freely with each other through 



