DIGESTION OF POOD. 333 



of these muscles also produces lateral movement of tlie inferior 

 maxilla, which is retracted by the more horizontal fibers of the 

 temporal. The movements of mastication are, of course, very 

 pronounced in ruminants. 



The cheeks and tongue likewise take part in preparing the 

 food for the work of the stomach, nor must the lips be over- 

 looked even in man. The importance of these parts is well 

 illustrated by the imperfect mastication, etc., when there is 

 paralysis of the muscles of which they are formed. Even when 

 there is loss of sensation only, the work of the mouth is done 

 in a clumsy way, showing the importance of common sensation, 

 as well as the muscular sense. 



Nervoiis Supply. — The muscles of the tongue are governed by 

 the hypoglossal nerve; the other muscles of mastication chiefly 

 by the fifth. The afferent nerves are branches of the fifth and 

 glosso-pharyngeal. It is, of coiu^e, important that the food 

 should be rolled about and thoroughly mixed with saliva (in- 

 salivation). 



Deglutition. — The transportation of the food from the mouth 

 to the stomach involves a series of co-ordinated muscular acts 

 of a complicated character, by which difliculties are overcome 

 with marvelous success. 



It will be remembered that the respiratory and digestive 

 tracts are both developed from a common simple tube — a fact 

 which makes the close anatomical relation between these two 

 physiologically distinct systems intelligible ; but it also involves 

 difficulties and dangers. It is well known that a small quantity i 

 of food or drink entering the windpipe produces a perfect ( 

 storm of excitement in the respiratory system. The food, there- i 

 fore, when it reaches the oesophagus, must be kept, on the one 1 

 hand, from entering the nasal, and on the other, the laryngeal j 

 openings. This is accomplished as follows : When the food has 

 been gathered into a bolus on the back of the tongue, the tip of 

 this organ is pressed against the hard palate, by which the 

 mass is prevented from passing forward, and, at the same time, 

 forced back into the pharynx, the soft palate being raised and 

 the edges of the pUlars of the fauces made to approach the 

 uvula, which fills up the gap remaining, so that the posterior 

 nares are closed and an inclined plane provided, over which 

 the morsel glides. The after-result is said to depend on the [ 

 size of the bolus. When considerable, the constrictors of the I 

 pharynx seize it and press it on into the gullet ; when the mor- 



