OXYGENATION OF BLOOD. SECT. XXXVlir. i. 



the lungs; and that by this addition the colour of 

 the blood is changed from a dark to a light red. 

 Secondly, that waier poilefies oxygene alfo as a part 

 of its compofition, and contains air likewife in its 

 p:>res; whence the blood of rifh receives oxygene 

 from the water, or from the air it contains, by 

 means of their gills, in the fame manner as the 

 blood is oxygenated in the lungs of air-breathing 

 animals; it changes its colour at the fame time 

 from a dark to a light red in the vefTels of their 

 gills, which- constitute a. pulmonary organ adapted 

 TO the medium in which they live. Thirdly, that 

 the placenta- confifts of arteries carrying the blood 

 to its extremities, and a vein bringing it back, rc- 

 fembling exactly in ftruture the lungs and gills 

 above mentioned ; and that the blood changes ife 

 colour from a dark to a light red in palling through 

 thefe veifels. 



This! analogy between the lungs and gills of ani- 

 mals', and the placenta of the fetus, extends through 

 a great variety of other circumflances ; thus air- 

 breathing creatures and fifh can live but a few mr- 

 nutes without air or water ; or when they are con- 

 fined in fuch air or water, as has been fpoiled by 

 rheir own refpiration ; the fame happens to the 

 fetus, which, as foon as the placenta is feparated 

 from the uterus, muft either expand its lungs, and 

 receive air, or die. Hence from the ftructure, as 

 well as the ufe of tire placema, it appears to be 

 a refpiratory organ, like the gills of fifh, by which 

 the blood in the fetus becomes oxygenated. 



From the terminations of the placemal veffels not 

 being obferved to bleed after being torn from the 

 uterus, while thofe of the uterus effufe a great quan- 

 tity of florid arterial blood, the terminations of the 

 plaeental veffels would feem to be inferted into the 

 arterial ones of the mother 5 and to receive- oxyge- 



natioa 



