134 COMPARATIVE PHYSIOLOGY. 



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 

 (impulses) which, passing along the nerves to the generative 

 organs, 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 

 center, the ejaculatory center, etc. — ^but we much doubt whether 

 there is such sharp division of physiological labor as these 

 terms imply, and they are liable to lead to misconception ; ac- 

 cordingly, in the present state of our knowledge, we prefer to 

 speak of the sexual center, using even that term in a somewhat 

 broad sense. 



The effects of stimulation of the sexual organs are not con- 

 fined to the parts themselves, but the ingoing impulses set up 

 radiating outgoing ones, which affect widely remote areas of 

 the body, as is evident, especially in the vascular changes; the 

 central current of nerve influence breaks up into many streams 

 as a result of the rapid and extensive rise of the outflowing 

 current, which breaks over ordinary barriers, and takes paths 

 which are not properly its own. Bearing this fact in mind, 

 the chemical composition of semen, so rich in proteid and other 

 material valuable from a nutritive point of view, and consid- 

 ering how the sexual appetites may engross the mind, it is not 



