THE SALIVARY GLANDS. 673 



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 necessary to 

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



An interesting fact with reference to the secretion of the parotid in dogs 

 has been noted by Langley, and is of special interest, since, although it may 

 be reconciled with the theory of trophic and secretory fibers, it is at the same 

 time suggestive of an incompleteness in this theory. As has been said, stimu- 

 lation of the sympathetic in the dog causes usually no secretion from the pa- 

 rotid. Langleyf finds, however, that, if the tympanic nerve is stimulated just 

 previously, stimulation of the sympathetic causes an abundant, but brief, 

 flow from the parotid. One may explain this in terms of the theory by as- 

 suming that the sympathetic does contain a few secretory fibers proper, but 

 that ordinarily their action is too feeble to start the flow of water. Pre- 

 vious stimulation of the tympanic nerve, however, leaves the gland cells 

 in a more irritable condition, so that the few secretory fibers proper in the 

 sympathetic branches are now effective in producing a flow of water. 



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

 They may be supposed to set up metabolic changes in the protoplasm 

 of the cells, leading to the formation of certain definite products, such 

 as mucin or ptyalin. That such changes do occur is abundantly 

 shown by microscopical examination of the resting and the active 

 gland, the details of which will be given presently. In general, these 

 changes may be supposed to be catabolic in nature; that is, to consist 

 in a disassociation or breaking down of the complex living material, 

 with the formation of the simpler and more stable organic con- 

 stituents of the secretion. There is evidence to show that these 

 gland cells during activity form fresh material from the nourishment 

 supplied by the blood ; that is, that anabolic or building-up processes 

 occur along with the catabolic changes. The latter are the more 

 obvious, and are the changes which are usually associated with the 

 action of the trophic nerve fibers. It is possible, also, that the ana- 

 bolic or growth changes may be under the control of separate fibers, 

 for which the name anabolic fibers would be appropriate. Satis- 

 factory proof of the existence of a separate set of anabolic fibers has 

 not yet been furnished. 



The method of action of the secretory fibers proper is difficult to 

 understand. At present the theories suggested are entirely specula- 

 tive. Heidenhain's own view rests upon the fact that no more water 

 leaves the blood capillaries than afterwards appears in the secretion; 

 that is, no matter how long the secretion continues, the gland does 

 not become edematous nor does the velocity of the lymph-stream in 

 the lymphatics of the gland increase. This being the case, we must 

 suppose that the stream of water is regulated by the secretion, that 

 is, by the activity of the gland-cells. If we suppose that some con- 



*" Journal of Physiology," 1, 96, 1878. j Ibid., 10, 291, 1889. 



43 



