THE DIGESTIVE APPARATUS AND NUTRITION 239 



rial introduced into the stomach may for a time relieve 

 the hunger, as will a very small quantity of food. The 

 special sensation of hunger appears to be connected with 

 the state of the lining membrane of the stomach, while it 

 must be ascribed in a more general sense to a deficiency 

 of nutrient matter in the blood. Hunger may be alle- 

 viated by the introduction of soluble food into the circula- 

 tion, through the rectum, or through the absorbents of the 

 skin, but the relief comes more slowly thus than through 

 the stomach. 



The nervous path of hunger sensations has not been 

 made out. The vagus is regarded as the sensory nerve 

 of the stomach, but it is said that both vagus nerves may 

 be cut and the sensation of hunger be unaffected. The 

 brain centers for thirst and hunger are believed to be in 

 the occipital lobes of the cortex, but they have not been 

 definitely located. 



346. Some Practical Points connected with Nutrition. In 

 order that the first of the digestive operations may be 

 properly performed, it is necessary to have a good set of 

 teeth and to chew thoroughly the food taken into the 

 mouth. The intensely hard enamel covering the exposed 

 portion of the teeth is a full protection to them against 

 all dangers .under proper conditions of life, and under 

 such conditions the teeth would last while life lasts. 

 That this is true is shown by the fact that nature makes 

 no provision for restoring or improving the enamel after 

 it is once formed. Here alone the special cells, whose 

 office it is to form the peculiar substance, entirely disap- 

 pear when their work is once completed. In all the other 

 tissues these formative cells remain to continue the nutri- 

 tion and repair of the tissues. But the tooth enamel, 

 whose growth, except in the wisdom teeth, is complete 



