764 PHYSIOLOGY OF DIGESTION AND SECRETION. 



experiment had been previously worked to a considerable extent, 

 then an increase in the stimulating current, while it augments the 

 amount of water and salts, either may have no effect at all upon the 

 organic constituents or may cause only a temporary increase, quickly 

 followed by a fall. Similar results may be obtained from stimulation 

 of the cerebral nerves of the parotid gland. The above facts led 

 Heidenhain to believe that the conditions determining the secretion 

 of the organic material are different from those controlling the water 

 and salts, and he gave a rational explanation of the differences 

 observed, in his theory of trophic and secretory fibers. 



Theory of Trophic and Secretory Nerve Fibers. This theory 

 supposes that two physiological varieties of nerve fibers are distrib- 

 uted to the salivary glands. One of these varieties controls the 

 secretion of the water and inorganic salts and its fibers may be called 

 secretory fibers proper, while the other, to which the name trophic 

 is given, causes the formation of the organic constituents of the secre- 

 tion, probably by a direct influence on the metabolism of the cells. 

 Were the trophic fibers to act alone, the organic products would be 

 formed within the cell, but there would be no visible secretion, and 

 this is the hypothesis which Heidenhain uses to explain the results of 

 the experiment described above upon stimulation of the sympathetic 

 fibers to the parotid of the dog. In this animal, apparently, the 

 sympathetic branches to the parotid contain exclusively or almost 

 exclusively trophic fibers, while in the cerebral branches both trophic 

 and secretory fibers proper are present. The results of stimulation 

 of the cerebral and sympathetic branches to the submaxillary gland 

 of the same animal may be explained in terms of this theory by 

 supposing that in the latter nerve trophic fibers preponderate, and 

 in the former the secretory fibers proper. 



It is obvious that this anatomical separation of the two sets of 

 fibers along the cerebral and sympathetic paths may be open to 

 individual variations, and that dogs may be found in which the sym- 

 pathetic branches to the parotid glands contain secretory fibers 

 proper, and therefore give some flow of secretion on stimulation. 

 These variations might also be expected to be more marked when 

 animals of different groups are compared. Thus, Langley* finds 

 that in cats the sympathetic saliva from the submaxillary gland is 

 less viscid than the chorda saliva, just the reverse of what occurs 

 in the dog. To apply Heidenhain's theory to this case it is necessaiy 

 to assume that in the cat the trophic fibers run chiefly in the chorda. 



The way in which the trophic fibers act has been briefly indicated. 

 They may be supposed to set up metabolic changes in the proto- 

 plasm of the cells, leading to the formation of certain definite prod- 



* "Journal of Physiology," 1, 96, 1878. 



