On The Parts Of Animals   

transversely in rows of more than two, one, that is, for each side of
the body, the right and the left; they must therefore be placed
lengthways, and the only place where there is sufficient length for
this is the region between the front and hind legs. As to the animals
that are not polydactylous but produce few at a birth, or have horns,
their dugs are placed in the region of the thighs. The horse, the ass,
the camel are examples; all of which bear but a single young one at a
time, and of which the two former have solid hoofs, while in the last
the hoof is cloven. As still further examples may be mentioned the
deer, the ox, the goat, and all other similar animals.
The explanation is that in these animals growth takes place in an
upward direction; so that there must be an abundant collection of
residual matter and of blood in the lower region, that is to say in
the neighbourhood of the orifices for efflux, and here therefore
nature has placed the mammae. For the place in which the nutriment is
set in motion must also be the place whence nutriment can be derived
by them. In man there are mammae in the male as well as in the female;
but some of the males of other animals are without them. Such, for
instance, is the case with horses, some stallions being destitute of
these parts, while others that resemble their dams have them. Thus
much then concerning the mammae.
Next after the breast comes the region of the belly, which is left
unenclosed by the ribs for a reason which has already been given;
namely that there may be no impediment to the swelling which
necessarily occurs in the food as it gets heated, nor to the expansion
of the womb in pregnancy.
At the extreme end of what is called the trunk are the parts concerned
in the evacuation of the solid and also of the fluid residue. In all
sanguineous animals with some few exceptions, and in all Vivipara
without any exception at all, the same part which serves for the
evacuation of the fluid residue is also made by nature to serve in
sexual congress, and this alike in male and female. For the semen is a
kind of fluid and residual matter. The proof of this will be given
hereafter, but for the present let it taken for granted. (The like
holds good of the menstrual fluid in women, and of the part where they
emit semen. This also, however, is a matter of which a more accurate
account will be given hereafter. For the present let it be simply
stated as a fact, that the catamenia of the female like the semen of
the male are residual matter. Both of them, moreover, being fluid, it
is only natural that the parts which serve for voidance of the urine
should give issue to residues which resemble it in character.) Of the
internal structure of these parts, and of the differences which exist
between the parts concerned with semen and the parts concerned with
conception, a clear account is given in the book of Researches
concerning Animals and in the treatises on Anatomy. Moreover, I shall
have to speak of them again when I come to deal with Generation. As
regards, however, the external shape of these parts, it is plain
enough that they are adapted to