350 FOOD AND DIGESTION 



an impression is conveyed upward (afferent) to the special nerve center 

 situated in the medulla oblongata which controls the process, and by it is 

 reflected to certain nerves supplied to the salivary glands, which will be pres- 

 ently indicated. In other words, the center, when stimulated to action by 

 the sensory impressions carried to it, sends out impulses along efferent or 

 secretory nerves supplied to the salivary glands. These cause the saliva to be 

 secreted by, and discharged from, the gland cells. Other stimuli, however, 

 besides that of the food, and other sensory nerves than those mentioned 

 may reflexly produce the same effects. For example, saliva may be caused 

 to flow by irritation of the mucous membrane of the mouth with mechanical, 

 chemical, electrical, or thermal stimuli, also by the irritation of the mucous 

 membrane of the stomach in some way, as in nausea which precedes vomit- 

 ing when some of the peripheral fibers of the vagi are irritated. Stimulation 

 of the olfactory nerves by smell of food, of the optic nerves by the sight of it, 

 and of the auditory nerves by the sounds which are known by experience to 

 accompany the preparation of a meal may also stimulate the nerve center to 

 action. In addition to these, as a secretion of saliva follows the movement 

 of the muscles of mastication, it may be assumed that this movement stimu- 

 lates the secreting nerve fibers of the gland, direct or reflexly. From the fact 

 that the flow of saliva may be increased or diminished by mental states, it 

 is evident that impressions from the cerebrum also are capable of stimulating 

 the center to action or of inhibiting its action. 



Influence of Nerves on the Submaxillary Gland. The submaxillary 

 gland has been the gland chiefly employed for the purpose of experimentally 

 demonstrating the influence of the nervous system upon the secretion of 

 saliva, because of the comparative facility with which the gland, with its blood 

 vessels and nerves, can be exposed to view in the dog, rabbit, and other 

 animals. 



The chief nerves supplied to the gland are: (i) the chorda tympani, a 

 branch given off from the facial in the canal through which it passes in the 

 temporal bone; and (2) branches of the sympathetic nerve from the plexus 

 around the facial artery and its branches to the gland. The chorda, figure 

 250, passes downward and forward, under cover of the external ptery- 

 goid muscle, and joins the lingual or gustatory nerve, proceeds with it for a 

 short distance, and then passes along the sub maxillary- gland duct, 

 giving branches to the submaxillary ganglion, and sending others to 

 terminate in the superficial muscles of the tongue. It consists of fine medul- 

 lated fibers which lose their medullae in the gland. If this nerve be exposed 

 and divided anywhere in its course from its exit from the skull to the gland no 

 immediate result will follow, nor will stimulation either of the lingual or of 

 the glosso-pharyngeal produce a flow of saliva. But if the peripheral end 

 of the divided nerve be stimulated, an abundant secretion of saliva ensues, 

 and the blood supply is enormously increased by dilatation of the arteries. 



