214 DIGESTION. 



active, even, than the saliva. The saliva acts slowly and imperfectly on raw starch, 

 which becomes hydrated in the stomach and is digested mainly hy the fluids of the small 

 intestine. In all probability, the saliva does not digest all the hydrated starch taken as 

 food, the greater part passing unchanged from the stomach into the intestine. Those who 

 attribute merely a mechanical function to the saliva draw their conclusions entirely from 

 experiments on the lower animals, particularly the carnivora ; and it is evident that such 

 observations cannot be strictly applied to the human subject. 



The principle which is specially active in the digestion of starch, in the human subject 

 at least, must exist in the pure secretion from the various glands as well as in the mixed 

 saliva. It has been isolated and studied by Mialhe, under the name of animal diastase. 

 Its properties and its action on starch have already been noted in treating of the com- 

 position of the mixed saliva. 



In treating of the various fluids which are combined to form the mixed saliva, their 

 mechanical functions have necessarily been touched upon. To sum up this subject, how- 

 ever, it may be stated that the fluids of the mouth and pharnyx have quite as important an 

 office in preparing the food for deglutition and for the action of the juices in the stomach 

 as in the digestion of starch. Indeed, the former is probably the more important function 

 in man and the herbivora. It is a matter of common experience that the rapid degluti- 

 tion of very dry articles is impossible ; and the experiments of Bernard and others on 

 horses furnish very striking illustrations of the importance of the snliva as a purely me- 

 chanical agent. In the human subject, although mastication and insalivation are by no 

 means so complete as in some of the lower animals, the quantity of saliva absorbed by the 

 various articles of food is enormous. It seems impossible that the fluid thus incorporated 

 with the alimentary principles should not have an important influence on the changes 

 which take place in the stomach, although it must be confessed that our information on 

 this point is very meagre, except as regards the digestion of starch. 



It is undoubtedly the abundant secretion of the parotid glands which becomes most 

 completely incorporated with the food during mastication and which serves to unite the dry 

 particles into a single coherent mass. In an experiment on a horse,. Bernard found that, 

 after the ducts of Steno had been divided, the portions of food, which were collected by 

 an opening into the oesophagus as they were swallowed, were not coherent and were 

 passed into the stomach with great difficulty. The time occupied in eating about three- 

 quarters of a pound of oats was twenty-five minutes; while, before the section of the 

 salivary ducts, a pound of oats was eaten in nine minutes. 



The secretions from the submaxillary and sublingual glands and from the small glands 

 and follicles of the mouth, being more viscid and less in quantity than the parotid secre- 

 tion, penetrate the alimentary bolus less easily and have rather a tendency to form a glairy 

 coating on its exterior, agglutinating the particles on the surface with peculiar tenacity. 



When the process of mastication and insalivation is completed and the food is passed 

 back into the pharynx, it meets with the secretion of the pharyngeal glands, which still 

 farther coats the surface with the viscid fluid which covers the mucous membrane in this 

 situation, thus facilitating the first processes of deglutition. 



It has been observed that the saliva has a remarkable tendency to entangle bubbles of 

 air in the alimentary mass. In mastication, a considerable quantity of air is mixed with 

 the food, and this undoubtedly facilitates the penetration of the gastric juice. It is well 

 known that moist, heavy bread, and articles that cannot become impregnated in this way 

 with air, are not easily acted upon in the stomach. 



Deglutition. 



Deglutition is the act by which solid and liquid articles are forced from the mouth 

 into the stomach. The process involves first, the passage, by a voluntary movement, of 

 the alimentary mass through the isthmus of the fauces into the pharynx ; then a rapid 

 contraction of the constrictors of the pharynx, by which it is forced into the oesophagus ; 



