296 SECRETION IN GENERAL 



excited by causes acting directly upon the nervous centers, upon the nerves 

 going to the secreting organ, or upon the nerves of other parts. In the latter 

 case a reflex action is produced: thus the impression produced upon the sen- 

 sory nerves by the contact of food in the mouth leads to afferent nerve impulses 

 to the secretory center in the central nervous system which is reflected by the 

 nerves supplying the salivary glands, and produces, through these, a more 

 abundant secretion of the saliva. 



Through the nerves, various conditions of the brain also influence the 

 secretions. Thus, the thought of food may be sufficient to excite an abundant 

 flow of saliva. And, probably, it is the mental state which excites the abun- 

 dant secretion of urine in hysterical paroxysms, as well as the perspirations, 

 and occasionally diarrhea, which ensue under the influence of terror, and the 

 tears excited by sorrow or excess of joy. The quality of a secretion may also 

 be affected by mental conditions, as in the cases in which, through grief or 

 passion, the secretion of milk is altered, and is sometimes so changed as to 

 produce irritation in the alimentary canal of the child. 



