concerning Animal Impregnation, 171 
and was retained; but when it returned from the vagina, then 
the animal remained unimpregnated. In this latter case, he 
supposes the semen had never passed beyond the vagina ; for 
if it had, he says it would have been retained. This argument 
he thinks is unanswerable. 
The insufficiency of this reasoning did not escape the pene- 
tration of his opponents ; and the immense mass of counter- 
facts poured out against him, like an irresistible torrent, bore 
away the very foundation of his doctrine. This brings the ad- 
vocates for the necessity of the contact of semen with the 
ovaries into a dilemma, from which they attempt to extricate 
themselves by contending, that fecundation does not require 
the application of semen to the ovaries in a palpable form; but 
that there is exhaled from it a subtile fluid in a vaporific state, 
called aura seminalis, and that the contact of this vapour is 
fully sufficient to impart to the ovaries their due quantity of 
stimulus. 
But the opinion, even thus qualified, has not passed without . 
animadversion. There are some who cannot comprehend how 
the tubes should perform two motions in contrary directions, 
which they must do, if they first convey the aura seminalis to 
the surface of the ovaries, and afterwards return the rudiments 
of the foetus into the uterus. Such a double action they think 
is repugnant to the oeconomy of the part, but assign no reason 
for their opinion. They might with equal propriety deny the 
possibility of a peristaltic and inverted peristaltic motion of the 
intestines, or the opposite actions in the oesophagus of rumi- 
nant animals, though I am persuaded very few would acquiesce 
in their incredulity: but as a minute discussion of this particular 
Za 
