FUNCTIONS OF THE SALIVA. 139 



I. That the manufacture of the specific materials of the 

 secretion is accomplished as the result of the intrinsic 

 power of the protoplasm of the gland cells. 

 II. That a vital process is called forth in the gland cells by 

 the action of nerve impulses, because (1.) The force 

 with which the secretion is expelled cannot be ac- 

 counted for by the blood pressure.- (2.) The quantity 

 and quality of the secretion is modified by the inten- 

 sity of the nerve stimulation. (3.) The temperature 

 of the blood is raised. (4.) Structural changes in the 

 cells can be observed. 



III. The normal stimulus to secretion passes from the centre 



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 energetic 

 action by impulses coming from taste, smell, and or- 

 dinary sensory nerve terminals (particularly in the 

 mouth), as well as by those which emanate from men- 

 tal 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 swal- 

 lowed, it is much aided by insalivation, particularly in chewing 

 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 accom- 

 plished 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 sup- 

 ply of saliva. 



