694 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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. 



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- 

 ucts, 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, they consist in a disassociation or breaking down of the 

 complex living material, with the formation of the simpler and 

 more stable organic constituents of the secretion. That these 

 changes involve processes of oxidation is shown by the fact that 

 during activity the gland takes up more oxygen and gives off more 

 carbon dioxid. 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 anabolic 

 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. Experiments have shown that the amount of water given 

 off from the blood during secretion is somewhat greater than the 

 amount contained in the saliva, f and there is reason to believe that 

 the difference between the two is accounted for by an increase in 

 the flow of lymph from the gland during activity. A satisfactory 

 explanation of the causes leading to and controlling the flow of 

 water cannot yet be given. In a general way we may believe that 

 the effect of the nerve impulses is to cause the production of sub- 

 stances within the cells whereby their osmotic pressure is increased, 

 and a stream of water is set up from the blood in the capillaries 

 toward the gland cells. The activity of the cells themselves initiates 

 and controls the flow of water. 



Histological Changes during Activity. The cells of both the 

 albuminous and mucous glands undergo distinct histological changes 



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



tBarcroft, "Journal of Physiology," 1900, xxv., 479. 



