THE PHYSIOLOGY OF DEGLUTITION. 205 



within its bubbles, and thus conveying a certain portion of 

 the atmosphere, or of oxygen and nitrogen gases, to the 

 stomach. Salivary glands, moreover, to maintain their 

 health, require to be pressed by the adjacent muscles ; 

 pressure being as necessary to the health of a gland of this 

 description, as food is to the support of the body. 



THE PHYSIOLOGY OF DEGLUTITION. 



When a horse grazes, he first places with his lips a tuft 

 between the teeth ; when, elevating his chin, the sharp 

 edges of the under incisors become applied to the grass, 

 and cut it through, while at the same time the upper in- 

 cisors help to nip it off. Oxen and sheep, wanting upper 

 nippers, wrap a tuft of grass round with their tongue, and 

 then apply it to the under incisors, by which it is held fast, 

 while a movement of the head tears the grass up ; and thus 

 we find that they are obliged always to carry the chin for- 

 ward in collecting their grass. When a quantity of herbage 

 is thus gained, it is carried by the tongue and muscles of 

 the cheeks to the upper part of the mouth, to encounter 

 the action of the molars : the matter being conveyed from 

 side to side, to be placed in the most favourable direction 

 for perfect mastication, by means of the tongue. During 

 this process, it continues to be mixed with the salivary 

 tluid, from the parotid, the submaxillary, and the sublin- 

 gual glands, which pour out their secretions, excited by the 

 pressure of the surrounding muscles. 



The vegetable mass having been thus completely masti- 

 cated, is placed at the back of the tongue ; when by the 

 pressure of that organ towards the palate it is squeezed 

 against the velum palati. The soft palate readily yields to 

 force coming from the mouth. The morsel is at the same 

 time driven against the epiglottis, which by its own elas- 

 ticity released from the downward pressure of the velum 

 palati, is raised, and covers the opening to the larynx. 

 The food is thus propelled into the fauces. Then the 

 larynx, protected by the epiglottis, rises and urges the 

 mass into the pharynx ; the contractors of which transmit it 

 to the oesophagus ; the muscular coats of which, contracting 

 as it descends, ultimately lodge it within the stomach. The 

 mouth remains closed during the act of swallowing, that 



