352 FOOD AND DIGESTION 



saliva ensues for a short time, although the blood supply is necessarily absent. 

 These experiments serve to prove that the chorda contains two sets of nerve 

 fibers: one set, V as o- dilator , which, when stimulated, act upon a local vaso- 

 motor center for regulating the blood supply, inhibiting its action, and 

 causing the vessels to dilate, and so producing an increased supply of blood 

 to the gland; while another set, which are paralyzed by injection of atropine, 

 directly stimulate the cells themselves to activity, whereby the cells secrete 

 and discharge the constituents of the saliva which they produce, the secretory 

 nerves. These latter fibers very possibly terminate on the salivary cells 

 themselves. If, on the other hand, the sympathetic fibers be divided, stimu- 

 lation of the tongue by sapid substances, or electrical stimulation of the trunk 

 of the lingual or of the glosso-pharyngeal, continues to produce a flow of 

 saliva. From these experiments it is evident that the chorda^ tympani nerve 

 is the principal nerve through which efferent impulses proceed from the center 

 to excite the secretion of this gland. 



The sympathetic nerve also contains two sets of fibers, v as o- constrictor 

 and secretory. But the flow of saliva upon stimulating the sympathetic is 

 scanty, and the saliva itself viscid. At the same time the vessels of the gland 

 are constricted. The secretory fibers may be paralyzed by the administra- 

 tion of atropine. 



Nerves of the Parotid Gland. The nerves which influence secretion 

 in the parotid gland are branches of the facial (lesser superficial petrosal) 

 and of the sympathetic. The former nerve, after passing through the otic 

 ganglion, joins the auriculo- temporal branch of the fifth cerebral nerve, and, 

 with it, is distributed to the gland. The nerves by which the stimulus ordi- 

 narily exciting secretion is conveyed to the medulla oblongata are, as in the 

 case of the submaxillary gland, the fifth and the glosso-pharyngeal. The 

 pneumogastric nerves convey a further stimulus to the secretion of saliva 

 when food has entered the stomach; the nerve center is the same as in the case 

 of the submaxillary gland. 



Changes in the Gland Cells. The method by which the salivary 

 cells produce the secretion of saliva appears to be divided into two stages, 

 which differ somewhat according to the class to which the gland belongs, viz., 

 whether to (i) the true salivary or to (2) the mucous type. In the former 

 case it has been noticed, as already described, that during the rest which 

 follows an active secretion the lumen of the alveolus becomes smaller, the 

 gland cells larger and very granular. During secretion the alveoli and their 

 cells become smaller, and the granular appearance in the latter to a consider- 

 able extent disappears, and at the end of secretion the granules are confined 

 to the inner part of the cell nearest to the lumen, which is now quite distinct, 

 figure 251. 



It is supposed from these appearances that the first stage in the act of 

 secretion consists in the protoplasm of the salivary cell taking up from the 



