124 ANIMAL PHYSIOLOGY. 
of the ciliated cells lining the oviduct. It is known that the 
male cell can survive in the female organs of generation for 
several days, a fact not difficult to understand, from the method 
of nutrition of the female cell (ovum); for we may suppose 
that both elements are not a little alike, as they are both 
slightly modified ameeboid organisms. 
Nervous Mechanism.—Incidental reference has been made to 
the directing influence of the nervous system over the events 
of reproduction; especially their subordination one to another 
to bring about the general result. These may now be consid- 
ered in greater detail. 
Most of the processes in which the nervous system takes 
part are of the nature of reflexes, or the result of the automa- 
ticity (independent action) of the nerve-centers, increased by 
some afferent (ingoing) impressions along a nerve-path. It is 
not always possible to estimate the exact share each factor 
takes, which must be highly variable. Certain experiments 
have assisted in making the matter clear. It has been found 
that, if in a female dog, the spinal cord be divided when the 
animal is still a puppy, menstruation and impregnation may 
occur. If the same experiment be performed on a male dog, 
erection of the penis and ejaculation of semen may be caused 
by stimulation of the penis. As the section of the cord has left 
the hinder part of the animal’s body severed from the brain, 
the creature is, of course, unconscious of anything happening 
in all the parts below the section, of whatever nature. If the 
nervi erigentes (from the lower part of the spinal cord) be 
stimulated, the penis is erected; and if they be cut, this act be- 
comes impossible, either reflexly by experiment or otherwise. 
Seminal emissions, it is well known, may occur during sleep, 
and may be associated, either as result or cause, with voluptu- 
ous dreams. Putting all these facts together, it seems reason- 
able to conclude that the lower part of the spinal cord contains 
the nervous machinery requisite to initiate those influences (im- 
pulses) which, passing along the nerves to the generative or- 
gans, excite and regulate the processes which take place in 
them. In these, vascular changes, as we have seen, always 
play a prominent part. 
Usually we can recognize some afferent influence, either 
from the brain (psychical), from the surface—at all events 
from without that part of the nervous system (center) which 
functions directly in the various sexual processes. It is com- 
mon to speak of a number of sexual centers—as the erection 
