FOOD AND DIGESTION 55 



behind, these cavities are often unprotected and some of 

 the ejected matter is forced into them. 



Nervous Mechanism. The nervous mechanism is a 

 very complex reflex action. A vomiting center may ex- 

 ist in the medulla. Many sensory nerves may be con- 

 cerned, particularly those connected with vision and equi- 

 librium. Irritation of the sensory nerves of the stomach 

 is, however, the most constant cause. 



Hunger and Thirst. These sensations are the cry of 

 cells throughout the body for food and drink, with a 

 local manifestation in the stomach and throat. The 

 empty stomach contracts at irregular intervals and these 

 contractions are attended by slightly painful sensations, 

 "hunger pains," until food or some substitute is taken. 

 Thirst is felt whenever the total amount of water in the 

 body falls beloAV a certain point. It occurs, therefore, 

 as a symptom following loss of blood, perspiration or fre- 

 quent urination. The sense of distress is felt in the 

 pharynx chiefly and is transmitted by the ninth cranial 

 (glossopharyngeal) nerve. 



Action of Digestive Secretions 



The Salivary Glands. These are three in number, 

 parotid, much the largest, situated between the lower 

 jaAv and the base of the skull; submaxillary, under the 

 body of the lower jaw, and the sublingual, just under the 

 tongue. The mixed secretion of these glands contains 

 994 parts of water and only six parts of solid material 

 in a thousand parts. The solids are some salts of potash, 

 soda and magnesia, mucin and ptyalin. 



Nervous Mechanism. As the nervous control of these 

 glands is well understood and is very similar to other 

 complex reflexes, it is explained at some length as an 

 example. The simplest form of reflex arc is a spot in 



