NERVOUS SYSTEM AND SECRETION 275 



nerve endings through the medium of digestible substances present 

 in the stomach, or through nerves of special sense by the sight of 

 appetising food, as has been shown by the experiments of Pawlow. 



In all cases, the efferent nervous impulses by which secretion 

 is excited pass along one of two paths, one coming directly from 

 the central nervous system and the other indirectly through the 

 sympathetic nervous system. 



In the case of the salivary glands, our knowledge of the efferent 

 paths belongs to classical and well-established physiological history, 

 while in the case of the gastric and pancreatic secretions the 

 efferent channels may be said still to be in dispute. 



The Innervation of the Salivary Glands. Each of the important 

 paired salivary glands receives efferent fibres from two sources 

 viz., directly from a cephalic nerve, and indirectly from the sympa- 

 thetic system. About eighty years ago Carl Ludwig showed 

 in the case of the submaxillary gland that the gland possessed a 

 special secretory nerve, the chorda tympani, which on stimulation 

 called into activity a copious secretion of saliva. The flow of 

 saliva was large in quantity but poor in organic constituents and 

 in the specific ferment. Some twenty-five years later Heidenhain 

 demonstrated that the gland also received secretory fibres from 

 the cervical sympathetic, which evoked a flow of saliva small in 

 total quantity but rich in organic constituents and in the specific 

 ferment produced by the gland. 



As a result of his experiments, Heidenhain evolved the theory 

 that the salivary glands possessed two sets of secretory fibres, 

 one obtained from the cephalic nerve and possessing the property 

 of evoking a flow of water and saline, and the other, which he 

 termed the " trophic " or " anabolic " nerve, obtained from the 

 sympathetic system and responsible for stimulating the secretion 

 of organic substances and the specific ferment. 



This view of Heidenhain's was subsequently generalised for 

 secretion in general without adequate experimental proof. In 

 the case of the submaxillary gland, however, it must be admitted 

 from the clear experimental evidence, that of the two efferent sets 

 of fibres which govern the secretion of the gland, one induces a 

 free flow of dilute saliva poor in organic constituents, while the 

 other causes a scanty flow of a richer saliva. Also, as shown by 

 Langley, alternate stimulation of the two nerves causes an increase 

 in the amount of saliva which would arise from stimulation of the 



