336 THE HUMAN BODY. 



ferments or enzymes. They differ from the true ferments, 

 such as yeast, in the fact that they are not living organisms, 

 and do not multiply during the occurrence of the change 

 which they set up; their activity belongs to the obscure 

 chemical category of contact actions. 



In order that the ptyalin may act upon starch certain 

 conditions are essential. Water must be present, and the 

 liquid must be neutral or feebly alkaline; acids retard, or if 

 stronger, entirely stop the process. The change takes place 

 most quickly at about the temperature of the Human Body, 

 and is greatly checked by cold. Boiling the saliva destroys 

 its ptyalin and renders it quite incapable of 'converting 

 starch. Boiled starch is changed more rapidly and com- 

 pletely than raw. 



Saliva has another important but indirect influence ii> 

 promoting digestion. Weak alkalies stimulate the mucous 

 membrane of the stomach and cause it to pour forth more 

 gastric juice. Hence the efficacy of a little carbonate of 

 soda, taken before meals, in same forms of dyspepsia. The 

 saliva by its alkalinity exerts such an action; and this 

 is one reason why food should be well chewed before being 

 swallowed; for then its taste, and the movements of the 

 jaws, cause the secretion of more saliva. 



Deglutition. A mouthful of solid food is broken up by 

 the teeth, and rolled about the mouth by the tongue, until 

 it is thoroughly mixed with saliva and made into a soft pasty 

 mass. The muscles of the cheeks keep this from getting 

 between them and the gums; persons with facial paralysis 

 have, from time to time, to press out with the finger food 

 which has collected outside the gums, where it can neither 

 be chewed nor swallowed. The mass is finally sent on from 

 the mouth to the stomach by the process of deglutition, 

 which is described as occurring in three stages. The first 

 stage includes the passage from the mouth into the pharynx. 

 The food being collected into a heap on the tongue, the tip 

 of that organ is placed against the front of the hard palate 

 and then the rest of its dorsum is raised from before back, 

 so as to press the food mass between it and the palate, and 

 drive it back through the fauces. This portion of the act of 



