424 DIGESTION 



for the secretion, and therefore vasodilatation alone can not be respon- 

 sible for it. If the sympathetic nerve supply is stimulated, a very scanty, 

 thick secretion takes place accompanied by vasoconstriction. 



Repetition of these experiments in the cat yields different results, 

 particularly with regard to the influence of the sympathetic, a copious 

 secretion being produced by stimulation of this nerve. The histological 

 changes produced in the gland cells are marked after sympathetic stimula- 

 tion, but very slight, if present at all, after chorda stimulation. 



The outstanding conclusion which may be drawn from these results 

 is that two kinds of secretory activity are mediated through the nerves; 

 one causing a thin watery secretion, containing only a small percentage 

 of organic matter, and the other, a thick viscid secretion with a large 

 amount of organic material. To explain these differences the hypothe- 

 sis has been advanced that, there are really two kinds of secretory 

 fibers, called secretory and trophic, the former having to do with the 

 secretion of water and inorganic salts, and the latter with the secretion 

 of organic matter ; i. e., with the extrusion of the zymogen granules. 

 Certain authors (Langley) believe that such an hypothesis is unneces- 

 sary, and that the different results are dependent upon the concomitant 

 changes in the blood supply produced by stimulating one or other nerve. 



That there are really different kinds of true secretory fibers is, however, 

 evident from the following experiment. If the duct of the gland is 

 made to open on the surface of the cheek, secretion of saliva through 

 the fistula can be induced by placing various substances in the mouth, such 

 as meat powder or weak solutions of acid. When the experiment is per- 

 formed in such a way that the bloodflow through the gland can be observed, 

 it has been found that the saliva produced by the stimulation with the meat 

 powder contains a very much higher percentage of organic material than 

 that produced when hydrochloric acid is the stimulant, whereas the vascular 

 changes in the gland and the inorganic constituents of the saliva are the 

 same in both cases. Since stimulation of the chorda tympani causes the 

 secretion of a watery saliva, while that caused by stimulation of the 

 sympathetic is thick, it might be thought that the secretory fibers are 

 contained in the former and the trophic fibers in the latter nerve; that 

 this is not the case can be shown by a repetition of the above experiment 

 in animals from which the superior cervical ganglion has been removed. 

 The same results are obtained, indicating that the chorda tympani con- 

 tains both secretory and trophic fibers. 



