DEGLUTITION. 213 



right: for, although gastric fluid added to saliva appears to 

 arrest the action of the latter on starch, yet portions of saliva 

 mingled with food in mastication may, for some time after their 

 entrance into the stomach, remain unneutralized by the gastric 

 secretion, and continue their influence upon the starchy prin- 

 ciples in contact with them. 



Starch appears to be the only principle of food upon which 

 saliva acts chemically : it has no apparent influence on any of 

 the other ternary principles, such as sugar, gum, cellulose, or 

 (according to Bernard) on fat, and seems to be equally desti- 

 tute of power over albuminous and gelatinous substances, so 

 that we have as yet no information respecting any purpose it 

 can serve in the digestion of Carnivora, beyond that of soften- 

 ing or macerating the food; though, since such animals masti- 

 cate their food very little, usually " bolting " it, the saliva has 

 probably but little use even in this respect, in the process of 

 digestion. 



Passage of Food into the Stomach. 



When properly masticated, the food is transmitted in suc- 

 cessive portions to the stomach by the act of deglutition or 

 swallowing. This act, for the purpose of description, may be 

 divided into three parts. In the first, particles of food col- 

 lected to a morsel glide between the surface of the tongue and 

 the palatine arch, till they have passed the anterior arch of 

 the fauces ; in the second, the morsel is carried through the 

 pharynx ; and in the third, it reaches the stomach through the 

 O3sophagus. These three acts follow each other rapidly. The 

 first is performed voluntarily by the muscles of the tongue and 

 cheeks. The second also is effected with the aid of muscles 

 which are in part endued with voluntary motion, such as the 

 muscles of the soft palate and pharynx ; but it is, nevertheless, 

 an involuntary act, and takes place without our being able to 

 prevent it, as soon as a morsel of food, drink, or saliva is 

 carried backwards to a certain point of the tongue's surface. 

 When we appear to swallow voluntarily, we only convey, 

 through the first act of deglutition, a portion of food or saliva 

 beyond the anterior arch of the palate ; then the substance acts 

 as a stimulus, which, in accordance with the laws of reflex 

 movements hereafter to be described, is carried by the sensi- 

 tive nerves to the medulla oblongata, when it is reflected by 

 the motor nerves, and an involuntary adapted action of the 

 muscles of the palate and pharynx ensues. The third act of 

 deglutition takes place in the oesophagus, the muscular fibres 

 of which are entirely beyond the influence of tlje will. 



The second act of deglutition is the most complicated, be- 



