DIGESTION. 363 



it is associated with the taking up of a molecule of water is 

 a hydrolytic action. Ptyalin is a typical enzyme ; it differs 

 from the true ferments, such as yeast, in the fact that it is 

 not a living organism, and does not multiply during the oc- 

 currence of the change which it sets up; its 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. 

 Cooked starch is changed more rapidly and completely than 

 raw. 



Saliva has another important but indirect influence in 

 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 some 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 " 

 the teeth, and rolled about the mouth by the tongue, untij 6 

 is thoroughly mixed with saliva and made into a soft pa& 

 mass. The muscles of the cheeks keep this from gettii 

 between them and the gums; persons with facial paralysL 

 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 in- 

 cludes 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 the tongue 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 swallow- 

 ing is voluntary, or at least is under the control of the will, 

 although it commonly takes place unconsciously. The second 



