44 PHYSIOLOGY FOR DENTAL STUDENTS. 



-be inhibition of the glandular activities on account of the pres- 

 ence of food products in the blood. Perhaps the most interesting 

 fact of all is that even a fasting animal will after a time fail to 

 salivate if he be repeatedly shown food which causes a secretion, 

 but which he is not permitted to get. The response is immedi- 

 ately established again, however, if some food, or indeed some 

 other object, be placed in the mouth. A hungry animal will even 

 salivate when he hears some sound which by previous experience 

 he has learned to associate with feeding time. The psychic 

 reflexes are evidently dependent upon an association of ideas (a 

 nervous integration, see p. 242) ; they are conditioned reflexes, 

 and are therefore the result of a certain degree of education. 

 They are easily rendered ineffective by confusing the usual asso- 

 ciations. 



General Functions of Saliva. These observations indicate 

 that a very important function of the saliva is what we may call 

 a mechanical one, namely, either to flood the mouth cavity with 

 fluid and so to wash away objectionable objects in it, or to lubri- 

 cate the food with mucin and so facilitate swallowing. The sol- 

 vent action of saliva is also important for the act of tasting (see 

 p. 295). Its chemical activities in many animals seem to be lim- 

 ited to the neutralizing properties of the alkali which is present 

 in it, but in man and the herbivora it also contains a certain 

 amount of a diastatic enzyme, ptyalin, which can quickly con- 

 vert cooked starches into dextrines and maltose. Even when this 

 action is most pronounced, however for it varies considerably 

 in different individuals it cannot proceed to any extent in the 

 mouth cavity, partly on account of the short time food remains 

 here, and partly because many starches, as in biscuits, are taken 

 more or less in a raw state. In some animals, such as the dog, 

 the saliva has no diastatic action whatever. Although there can 

 therefore be little diastatic digestion in the mouth, a good deal 

 may go on in the stomach, for the saliva that is swallowed along 

 with the food does not become destroyed by the gastric juice 

 until some thirty minutes after the food has gained the stomach. 



Although mastication of the food and its preparation for 

 swallowing are undoubtedly the main functions of the mouth cav- 



