136 COMPARATIVE PHYSIOLOGY. 



The blood of the fcetus (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 excre- 

 tory 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 

 peristaltic action of the various parts of the sexual tract, aided 

 by rhythmical action of certain striped muscles. The sperma- 

 tozoa, which are unicellular, flagellated (ciliated) cells, make up 

 the essential 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 move- 

 ments of reaching the ovum in the oviduct, it is probable that 

 the uterus and oviduct experience peristaltic actions ia a .direc- 

 tion 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 

 aiferent 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. 



