CHAP, i.] FEMALE ORGANS. 363 



are brought about. It is no more difficult to suppose that the 

 stimulus of the enlargement of a Graaffian follicle causes nutritive 

 as well as vascular changes in the uterine mucous membrane, than 

 it is to suppose that the stimulus of food in the alimentary canal 

 causes those nutritive changes in the salivary glands or pancreas 

 which constitute secretion. In the latter case we can to some ex- 

 tent trace out the chain of events ; in the former case we hardly 

 know more than that the maintenance of the lumbar cord is suffi- 

 cient, as far as the central nervous system is concerned, for the 

 carrying on of the work. In the case of a dog in which the spinal 

 cord had been completely divided in the thoracic region while the 

 animal was as yet a mere puppy, ' heat ' took place as usual. 

 And, though ' heat ' is something different from human menstrua- 

 tion the two are probably, in this respect, analogous. 



