232 HAND-BOOK OF PHYSIOLOGY. 



f erior maxillary division of the fifth nerve, and the glosso-pharyngeal part 

 of the eighth pair of nerves, which are distributed to the mucous mem- 

 brane of the tongue and pharynx. The stimulation occurs on the intro- 

 duction of sapid substances into the mouth, and the secretion is brought 

 about in the following way. From the terminations of these sensory 

 nerves in the mucous membrane an impression is conveyed upward (affer- 

 ent) to the special nerve centre situated in the medulla, which controls 

 the process, and by it is reflected to certain nerves supplied to the salivary 

 glands, which will be presently indicated. In other words, the centre, 

 stimulated to action by the sensory impressions carried to it, sends out 

 impulses along efferent or secretory nerves supplied to the salivary glands, 

 which 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 besides those mentioned, may produce reflexly the same effects. 

 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 vomiting, when some of the peripheral fibres 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 prepa- 

 ration of a meal, may also, in the hungry, stimulate the nerve centre to 

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

 ment of the muscles of mastication, it may be assumed that this move- 

 ment stimulates the secreting nerve fibres of the gland, directly or re- 

 flexly. From the fact that the flow of saliva may be increased or dimin- 

 ished by* mental emotions, it is evident that impressions from the cere- 

 brum also are capable of stimulating the centre to action or of inhibiting 

 its action. 



Secretion may be excited by direct stimulation of the centre in the 

 medulla. 



A. On the Submaxillary Gland. The submaxillary gland has been 

 the gland chiefly employed for the purpose of experimentally demonstra- 

 ting the influence of the nervous system upon the secretion of saliva, be- 

 cause of the comparative facility with which, with its blood-vessels and 

 nerves, it may be exposed to view in the dog, rabbit, and other animals. 

 The chief nerves supplied to the gland are: (1) the chorda tympani (a 

 branch given off from the facial portio dura of the seventh pair of nerves), 

 in the canal through which it passes in the temporal bone, in its passage 

 from the interior of the skull to the face; and (2) branches of the sym- 

 pathetic nerve from the plexus around the facial artery and its branches 

 to the gland. The chorda (Fig. 170, ch. t.), after quitting the temporal 

 bone, passes downward and forward, under cover of the external pterygoid 

 muscle, and joins at an acute angle the lingual or gustatory nerve, pro- 



