274 EXTRINSIC MECHANISMS OF SECRETION 



INFLUENCE OF THE NERVOUS SYSTEM UPON SECRETION. 



The profound influence of the nervous system upon secretion 

 is in the case of certain glandular structures a matter of common 

 experience. Thus it is well known to us that the sight or smell of 

 food often provokes salivary secretion, or causes the mouth to 

 water, in everyday parlance ; but the effect of the nervous system 

 is in the case of other glands most difficult to prove, and may be 

 said in certain cases not even yet to have been unequivocally 

 demonstrated. Certain it is in some cases from the recent ex- 

 periments of Bayliss and Starling that this nervous stimulation 

 cannot be regarded as the sole, if indeed the most fundamental 

 and important, factor; and we shall see in the next section that 

 it must be regarded as supplemented or replaced by the important 

 action of chemical stimulation and the production of specific 

 secretory substances which act upon the secreting cells after having 

 been absorbed by the circulating blood. 



As has been well pointed out by Pawlow, it is dangerous, in 

 the case of the nervous mechanisms of secretion, to generalise 

 from the somewhat simple mechanism of salivary secretion for 

 the secretory innervation of the whole secreting system of glands, 

 for in the case of other glands, such as the gastric glands and prob- 

 ably the pancreas, the influence of inhibitory nervous mechanisms 

 comes into play and complicates the problem. Hence we are 

 forced to consider separately the nervous mechanisms in the case of 

 each important secreting gland. 



Before proceeding to the separate accounts, however, it may 

 be well to point out the general resemblances. 



In each case where an influence of the nervous system upon 

 secretion has been clearly demonstrated, it has been shown that 

 a complete reflex arc exists. The nervous stimulation is excited 

 at the peripheral endings of afferent fibres, which excite nerve 

 cells in the central nervous system and cause stimuli to be dis- 

 charged along efferent paths to the secreting cells. In the case 

 of the salivary glands the afferent channels are nerves of special 

 sense, either the optic or ophthalmic nerves, or the endings of 

 the gustatory nerves in the mucous membrane of the mouth. In 

 the case of the gastric secretion the afferent impulses arise at the 

 mucous membrane of the stomach by the stimulation of peripheral 



