148 MANUAL OF PHYSIOLOGY. 



in the medulla oblongata to the salivary glands along 

 cerebro-spinal, not sympathetic, nerves. 

 IV. This centre for salivary secretion, which at ordinary 

 times is moderately active, may be excited to ener- 

 getic action by impulses coming from taste, smell and 

 ordinary sensory nerve terminals (particularly in 

 the mouth), as well as by those which emanate from 

 mental emotions. 



CHANGES UNDERGONE BY THE FOOD IN THE MOUTH. 



Food when taken in the mouth undergoes two processes, which 

 are inseparable and simultaneous in action ; viz., mastication and 

 insalivation. 



The mechanism of mastication has already been discussed, so 

 far as its triturating power is concerned. In its final object of 

 forming the subdivided food into a bolus which can be easily 

 swallowed, it is much aided by insalivation, particularly in chew- 

 ing dry food ; and in this latter the moistening of the particles, 

 so as to make them adhere together, is the most necessary act of 

 mouth digestion, and is next in importance to the subdivision 

 accomplished by the teeth. Moreover, the saliva covers the 

 bolus with a coating of viscid fluid, so that it can more easily 

 slip down the oesophagus. Deglutition of solids is difficult 

 during a scanty supply of saliva. 



Our food is generally composed of materials some of which 

 are soluble in water, and some are not. 



While in the mouth the saliva dissolves a great quantity of the 

 more readily soluble materials, such as sugar and salt, which may 

 be either mingled with the insoluble substances, and swallowed 

 together with the bolus, or swallowed separately in a fluid form. 

 Solution, then, is an important item in mouth digestion. 



In many carnivorous animals the use of the mouth fluid is 

 chiefly mechanical, dissolving some insignificant part of the food, 

 and aiding mastication and deglutition. In man, however, and 

 other animals that make use of much vegetable food, it has a 

 chemical. function, and acts on the insoluble starch, converting it 

 into soluble sugar. 



