THE SALIVARY GLANDS 261 



The salivary nerve centers are located in the medulla; for reflex secretion is 

 obtained after transsection of the brain in the pons. A puncture in the medulla 

 is followed likewise by secretion. Unilateral injury to the floor of the fourth 

 ventricle a little behind the origin of the trigeminal nerve causes secretion in 

 both submaxillary glands and in the parotid of the same side. Both cerebral 

 and sympathetic nerves are roused to activity in this case. It is possible that 

 the glands on each side of the body have their own centers, and that these are 

 connected together by commissures (Beck). 



The salivary glands can be set in action also by artificial stimulation of that 

 part of the cerebral cortex which corresponds roughly to the motor zone. It is 

 very probable that the above-mentioned psychical influence on the salivary glands 

 depends upon this cortical field. 



B. MORPHOLOGICAL CHANGES DURING SECRETION 



The more recent investigations of this subject have been made upon prac- 

 tically fresh material instead of, as formerly, upon preserved material. We 

 shall follow the descriptions given by Langley and by Biedermann. 



In the albuminous glands (Fig. 100) Langley found that in the resting 

 state, the cells are filled with a collection of granules so abundant as to ob- 

 scure the cell boundaries. When the gland has secreted for some time, the 

 cells increase in size, the granules gradually disappear especially from the 

 outer zone, or the side toward the membrana propria, while the inner zone 

 or the side toward the lumen of the gland still contains granules. These 

 changes are constant whether the gland be caused to secrete by the natural 

 stimulus, by injection of pilocarpine, or by stimulation of its nerves. 



According to E. Miiller, there occurs here a conversion of strongly refractive 

 granules into feebly refractive ones, which pass into the secretion as small 

 spherical drops the so-called secretion vacuoles (Fig. 101; cf. also Fig. 99). 

 In very active secretion the first-named granules pass directly over into the 



FIG. 101. Parotid gland of the cat, after E. Miller. Sublimate fixation. A, after twenty-four 

 hours' fast. B, during active discharge of the secretion. 



secretion vacuoles. When they leave the gland cells they pass first into the 

 secretory capillaries running between the cells where they are dissolved and 

 whence the secretion flows into the ducts of the gland. 



