90 HUMAN PHYSIOLOGY. 



The nerve centers exciting, through efferent nerves the secretion of saliva 

 are located in the medulla oblongata and may be aroused to action (i) by 

 nerve impulses descending from the brain in consequence of psychic states 

 induced by the sight and odor of food and (2) by nerve impulses reflected 

 through afferent nerves from the mouth developed by the taste of food. The 

 afferent nerves thus stimulated in the second instance are those stated in 

 the foregoing tabulation. 



That the efferent nerves in the same tabulation are active in the production 

 of the secretion is shown by the following facts: 



Stimulation of the auriculo temporal branch increases the flow of saliva 

 from the parotid gland; division arrests it. 



Stimulation of the chorda tympani is followed by a dilatation of the blood- 

 vessels of the submaxillary and sublingual glands, an increased flow of blood 

 and an abundant discharge of saliva; division of the nerve arrests the secre- 

 tion. 



Stimulation of the cervical sympathetic is followed by a contraction of the 

 blood-vessels, a diminished flow of blood, and a diminution of the secretion, 

 which now becomes thick and viscid; division of the sympathetic is not, 

 however, followed by complete dilatation of the vessels. There is evidence 

 of the existence of a local vasomotor mechanism, which is inhibited by the 

 chorda tympani. 



DEGLUTITION. 



Deglutition is the act of transferring food from the mouth into the stomach, 

 and may be divided into three stages: 



1. The passage of the bolus from the mouth into the pharynx. 



2. From the pharynx into the esophagus. 



3. From the esophagus into the stomach. 



In the first stage, which is entirely voluntary, the mouth is closed and res- 

 piration momentarily suspended; the tongue, placed against the roof of the 

 mouth, arches upward and backward, and forces the bolus into the fauces. 



In the second stage, which is entirely reflex, the palate is made tense and 

 directed upward and backward by the levatores palati and tensores palati 

 muscles; the bolus is grasped by the superior constrictor muscle of the phar- 

 ynx and rapidly forced into the esophagus. 



The food is prevented from entering the posterior nares by the uvula and 

 the closure of the posterior half -arches (the palatopharyngeal muscles) ; from 

 entering the larynx by its ascent under the base of the tongue and the action 

 of the epiglottis. 



In the third stage the longitudinal and circular muscle-fibers, contracting 

 from above downward, strip the bolus into the stomach. 



