126 ANIMAL PHYSIOLOGY. 
it and the foetus are finally expelled, so that these changes may 
constitute one of a number of exciting causes of the increased 
uterine action of parturition. But it is important to regard the 
whole of the occurrences of pregnancy as a connected series of 
processes co-ordinated by the central nervous system so as to 
accomplish one great end, the development of a new individual. 
The nutrition of the ovum in its earliest stages is effected by 
means in harmony with its nature as an amceboid organism; 
nutrition by the cells of blood-vessels is similar, while that by 
villi may be compared to what takes place through the agency 
of similar structures in the alimentary canal of the adult mam- 
mal. 
The circulation of the foetus puts it on a par physiologically 
with the lower vertebrates. Before birth there is a gradual 
though somewhat rapid preparation, resulting in changes 
which speedily culminate after birth on the establishment of 
the permanent condition of the circulation of extra-uterine life. 
The blood of the foetus (as in the adult) is the great store- 
house of nutriment and the common receptacle of all waste 
products; these latter are in the main transferred to the moth- 
er’s blood indirectly in the placenta; in a similar way nutri- 
ment is imported from the mother’s blood to that of the foetus. 
The placenta takes the place of digestive, respiratory, and ex- 
cretory organs. 
Coitus is essential to bring the male and female elements 
together in the higher vertebrates. The erection of the penis is 
owing to vascular changes taking place in an organ composed 
of erectile tissue; ejaculation of semen is the result of the peri- 
staltic action of the various parts of the sexual tract, aided by 
rhythmical action of certain striped muscles. The spermatozoa, 
which are unicellular, flagellated (ciliated) cells, make up the es- 
sential part of semen; though the latter is complicated by the 
addition of the secretions of several glands in connection with 
the seminal tract. Though competent by their own movements 
of reaching the ovum in the oviduct, it is probable that the 
uterus and oviduct experience peristaltic actions in a direction 
toward the ovary, at least in a number of mammals. 
The lower part of the spinal cord is the seat in the higher 
mammals of a sexual center or collection of cells that receives 
afferent impulses and sends out efferent impulses to the sexual 
organs. This, like all the lower centers, is under the control of 
the higher centers in the brain, so that its action may be either 
initiated or inhibited by the cerebrum. 
