DIGESTION 177 



backward to help the grinding motions of the teeth and to keep the 

 maxilla in its socket. The internal pterygoid muscle is in shape and 

 direction much like the masseter, and acts to strongly press the lower 

 jaw almost directly upward against the upper. The inferior maxil- 

 lary nerve supplies these three muscles. These three muscles combined 

 exert a pressure of several kilos as the crushing power of the molar teeth. 

 (2) The lateral mover of the inferior maxilla is the external pterygoid 

 muscle, whose powerful fibers pass horizontally backward and outward 

 to the condyle of the jaw. When the muscle of the right side contracts 

 the jaw is moved toward the left, and vice versa. Owing to the presence 

 of the fibers which pass backward, when both external pterygoids con- 

 tract together the jaw is drawn forward. This muscle also is supplied 

 by the inferior maxillary branch of the fifth nerve. (3) The muscles 

 which lower the jaw are the digastric, the mylo-hyoid, the genio-hyoid, 

 and the platysma, of which the first two are the more important, while 

 gravity is a force which in the mastication of ordinary food aids in the 

 descent of the jaw. 



The digastric muscle has two distinct bellies, the anterior and posterior, 

 and only the latter has action in depressing the jaw. This anterior belly 

 extends from the hyoid bone upward and forward to the inner surface 

 of the lower jaw near the median-line. It is supplied by the inferior 

 dental branch of the fifth nerve. The mylo-hyoid muscle is a flatter 

 muscle than the preceding, and is placed just inside it, forming the mus- 

 cular floor of the mouth. It extends between the hyoid bone and the 

 mylo-hyoid ridge of the inferior maxilla. The inferior dental nerve 

 supplies this muscle also. The genio-hyoid muscle is a slender bundle of 

 fibers extending between the symphysis of the jaw and the middle por- 

 tion of the hyoid bone. This muscle is supplied by the twelfth or hypo- 

 glossal nerve. When the hyoid bone is fixed from below by tonic con- 

 traction of the sterno-hyoid and other muscles, contraction of these 

 three muscles depresses the jaw with considerable force. The platysma 

 myoides muscle is a thin and superficial muscular fabric extending from 

 the clavicle and acromion and superficial fascia of the upper part of the 

 thorax to the whole length of the body of the jaw. Its contraction tends 

 to depress the jaw. 



Mastication employs these muscles, the tongue, and the muscles 

 of the cheeks, these last being the active agents in keeping the food 

 between the cutting and grinding teeth. In man the masticatory 

 movements are largely vertical, the antero-posterior motion being slight, 

 as are also the lateral movements of the jaw. As has been noted from 

 our rehearsal of the actions of the various muscles, a large variety of 

 jaw-movements are possible through the simultaneous operation of more 

 than one muscle each with a different effect on the jaw. By this means, 

 on a principle universal almost in muscular coordination, all the oblique 

 motions of the jaw are to be observed during vigorous mastication unre- 

 strained by the inhibiting conventionalities of culture. The tongue is at 

 once the master and the servant of the mouth, and pervades it on 

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