CHAP, i.] MENSTRUATION. 671 



changes of the organs concerned. Our studies on the nervous 

 action of secretion render it easy for us to conceive in a general 

 way how the several events 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 extent trace out the chain of events ; in 

 the former case we hardly know more than that the maintenance 

 of the lumbar cord is sufficient, 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 dorsal region while the animal was as yet a mere puppy, 'heat' 

 or menstruation took place as usual. 



