1NSALIVATION. 



179 



FIG. 103. A number of alveoli from 

 the submaxillary gland of dog, stained in 

 chrome-silver, showing some of the fine 

 intercellular tubules (Huber). 



From the mass of evidence which has been accumulated, to 

 only a small part of which have we made reference, it is indis- 

 putably proved that there are true secretory fibers distributed to 

 the salivary glands ; and the same is true of other glands as well, 

 such as the lachrymal, perspiratory, and gastric glands ; but the 

 method by which these fibers terminate is not so certain, though 

 it appears probable that it is by forming plexuses between 

 and around the secreting 

 cells. Further than this, it 

 is more than probable that 

 there are two kinds of se- 

 cretory fibers : One which 

 transmits impulses that cause 

 the secretion of the organic 

 constituents of the saliva, and 

 called trophic; the impulses 

 conveyed by the other, the 

 secretory fibers proper, pro- 

 ducing the secretion of the 

 water and the inorganic salts 

 of the saliva. 



Paralytic Secretion. If 

 the chorda tympani is cut, 

 there is no immediate effect, 

 but after a day or two the gland begins to secrete a very 

 watery saliva which continues for several weeks, and then it 

 atrophies. This secretion is called paralytic. Although the sec- 

 tion is made on but one side, both glands are similarly af- 

 fected, and the secretion on the opposite side to the section is 

 called antiparalytiG or antttytic. There are two explanations given 

 to account for this phenomenon. One is that the continuous 

 secretion is due to an increased irritability of the secretion-center 

 in the medulla oblongata and of the nerve-cells in the gland, by 

 which the venous condition of the blood is alone sufficient to cause 

 them to send out continuous impulses to the secreting cells. The 

 other theory is based upon the fact that katabolic changes are 

 going on in the cells of the gland which are inhibited by impulses 

 coming through the chorda tympani ; if, therefore, this nerve is 

 cut, such impulses no longer restrain the katabolic changes, and 

 they go on without interference, with the result of atrophy of the 

 gland and the formation of the paralytic secretion. 



Secretion of Saliva. The secretion of saliva is normally a 

 reflex act. It is ordinarily brought about by the stimulation of 

 the mucous membrane of the mouth by sapid substances that is, 

 those which excite the sense of taste, although chemical and even 

 mechanical stimuli will produce the same result. The chewing 

 of a piece of rubber will cause a profuse flow of saliva. The 



