2. OXYGEN ATION OF BLOOTJ. $37 



nation from the pafling currents of her blood 

 through their coats or membranes ; which oxyge- 

 nation is proved by the change of the colour of 

 the blood irom dark to light red in its paifage from 

 the placental arteries to the placental vein. 



The curious ftrudure of the cavities or lacunas 

 of the placenta, demonftrated by Mr. J. Hunter, 

 explain this eircumftance. That ingenious philofo* 

 pher has fliewn, that there are numerous cavities or 

 lacunae formed on that iide of the placenta, which 

 is in contact with the uterus ; thofe cavities or 

 cells are filled with blood from the maternal arte- 

 ries, which open into them ; which blood is again 

 taken up by the maternal veins, and is thus per- 

 petually changed. While the terminations of the 

 placental arteries and veins are fpread in tine reticu- 

 lation on the fides of thefe cells. And thus, as the 

 growing fetus requires greater oxygenation, an ap- 

 paratus is produced refembling exactly the air-cells 

 of the lungs. 



In cows, and other ruminating animals, the in- 

 ternal furface of the uterus is unequal like hollow 

 cups, which have been called cotyledons ; and into 

 thefe cavities the prominencies of the numerous pla- 

 centas, with which the fetus of thofe animals is 

 furnifhed, are inferted, and ftrictly adhere ; though 

 they may be extracted without efFufion of blood. 

 Thefe inequalities of the uterus, and the numerous 

 placentas in confequence, feern to be defigned for 

 the purpofe of expanding a greater furface for the 

 terminations of the placental veflels for the purpofe 

 of receiving oxygenation from the uterine ones; as 

 the progeny of this clafs of animals are more com- 

 pletely formed before their nativity, than that of 

 the carnivorous clafles, and muft thence in the lat- 

 ter weeks of pregnancy require greater oxygenation. 

 Thus calves and lambs can walk about in a few 



minutes 



