go Dr. Young's Lecture on the Functions 
in the extreme vessels being also probably much smaller than 
usual : if the motion was continued, it would lead us to ima- 
gine that there may be some structure in the placenta capable 
of assisting in the propulsion of the blood, as there may pos- 
sibly be some arrangement in the roots of plants by which 
they are calculated to promote the ascent of the sap. The 
circulation in the vessels of the more imperfect animals, in 
which a great artery supplies the place of a heart, is of a very 
different nature from that of the more perfect animals : the 
great artery, which performs the office of the heart, is here 
possessed of a muscular power commensurate to its functions, 
and seems to propel the blood, though much more slowly 
than in other cases, by means of a true peristaltic motion. It 
appears also from the observations of Spallanzani, that in 
many animals a portion of the aorta, next the heart, is capable 
of exhibiting a continued pulsation, even when perfectly empty 
and separated from the heart ; but this property is limited to 
a small part of the artery only, which is obviously capable of 
being essentially useful in propelling the blood when the 
valves of the aorta are closed. The muscular power of the 
termination of the vena cava is also capable of assisting the 
passage of the blood into the auricle. It is not at all impro- 
bable that a muscle of involuntary motion, which had been 
affected throughout the whole period of life by alternate con- 
tractions and relaxations, might retain from habit the tendency 
to such contractions, even without the necessity of supposing, 
that the habit was originally formed for a purpose to be ob- 
tained by the immediate exertion of the muscular power : 
but in fact the partial pulsation of the vena cava is perfectly 
well calculated to promote the temporary repletion of the 
