754 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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. 

 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 nourish- 

 ment supplied by the blood; that is, that anabolic or synthetic 

 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. 



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

 understand. At present the theories suggested are entirely specula- 

 tive. Ex:periments have shown that the amount of water given 

 off from the blood during secretion is somewhat greater than the 

 amount contained in the saliva,* 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 it has been assumed 

 that the effect of the nerve impulses is to cause the production 

 of substances 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, but it cannot be said that 

 this assumption has been supported by the experiments so far 

 made.f We must limit ourselves to the more general statement 

 that 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 in consequence of prolonged activity, and these changes 

 may be recognized both in preparations from the fresh gland 

 and in preserved specimens. In the parotid gland Heidenhain 

 studied the changes in stained sections after hardening in 

 alcohol. In the resting gland the cells are compactly fillea 

 with granules that stain readily and are imbedded in a clear 

 ground substance that does not stain. The nucleus is small and 

 more or less irregular in outline. After stimulation of the 

 tympanic nerve the cells show but little alteration, but stimula- 

 tion of the sympathetic produces a marked change. The cells 

 become smaller, the nuclei more rounded, and the granules more 

 closely packed. This last appearance seems, however, to be 



* Barcroft, "Journal of Physiology," 1900, 25, 479. 



t Carlson, Greer, and Becht, "American Journal of Physiology," 19, 360. 

 1907. 



