THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 195 
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 
difficult to understand that nothing so quickly disorganizes the 
whole man, physical, mental, and moral, as sexual excesses, 
whether by the use of the organs in a natural way, or from 
masturbation. 
Nature has protected the lower animals by the strong bar- 
rier of instinct, so that habitual sexual excess is with them an 
impossibility, since the females do not permit of the approaches 
of the male except during the rutting period, which occurs only 
at stated, comparatively distant periods in most of the higher 
mammals. When man keeps his sexual functions in subjection 
to his higher nature, they likewise tend to advance his whole 
development. ; 
Summary.—Certain changes, commencing with the ripening. 
of ova, followed by their discharge and conveyance into the 
uterus, accompanied by simultaneous and subsequent modifica- 
tions of the uterine mucous membrane, constitute, when preg- 
nancy occurs, an unbroken chain of biological events, though 
usually described separately for the sake of convenience. 
When impregnation does not result, there is a retrogression in 
the uterus (menstruation) and a return to general quiescence 
in all the reproductive organs. 
Parturition is to be regarded as the climax of a variety of 
rhythmic occurrences which have been gradually gathering 
head for a long period. The changes which take place in the 
placenta of a degenerative character fit it for being cast off, and 
may render this structure to some extent a foreign body before 
