found in the human Placenta . 3 6$ 
found to do in fishes. The junction between the two ventricles 
is attended with a great advantage, in performing the circula- 
tion through the placenta ; where the length and convolution 
of the umbilical vessels, in some animals, offer a great resistance 
to the force of the heart, and render more exertion necessary. 
I11 the superior aorta, the circulation is carried on by the left 
ventricle alone ; as the ductus arteriosus does not join the aorta, 
till after the latter has given off the carotid and subclavian 
branches. 
Vital air is communicated to the blood of the embryo, as it 
is to the blood of fishes. This, in its passage through the 
gills, is exposed to water, which is allowed by all to contain a 
large proportion of vital or oxygen gas, and returns thence 
fitted to answer the purposes of life. 
In like manner, the blood of the mother, in the cells of the 
placenta, having received the essential part of this gas from her 
lungs, is applied to the capillary vessels of the umbilical arte- 
ries, which receive and transmit it to the embryo ; the life of 
which so entirely depends upon this communication, that an 
obstruction to the circulation through the placenta, for the space 
of two or three minutes, will sometimes irrecoverably destroy it. 
The gills of fishes form a permanent part of their bodies ; 
because they are designed to pass the whole of their lives in 
the same medium. This is not the case in the embryo of vivi- 
parous animals ; which, after birth, is to change its situation 
for another, in which there is a direct exposure of the blood to 
atmospheric air. For this reason, the placenta, whose use is 
only temporary, is attached to the foetus by a slender connection, 
which is soon dissolved after birth. 
I have thought it necessary to introduce the foregoing obser- 
3 As 
