190 MYLO-HYOIDEUS GENIO-HYOIDEUS. 



Relations. By its superficial surface with the posterior belly of the 

 digastricus, the parotid gland and submaxillary gland ; its deep relations 

 are similar to those of the posterior belly of the digastricus. 



The digastricus and stylo-hyoideus must be removed from their con- 

 nexion with the lower jaw and os hyoides, and turned aside in order to 

 see the next muscle. 



The MYLO-HYOIDEUS (fxu'Xr), mola, i. e. attached to the molar ridge of 

 the lower jaw) is a broad triangular plane of muscular fibres, forming, 

 with its fellow of the opposite side, the inferior wall or floor of the mouth, 

 It arises from the molar ridge on the lower jaw, and proceeds obliquely 

 inwards to be inserted into the raphv of the two muscles and into the 

 body of the os hyoides ; the raphe is sometimes deficient at its anterior 

 part. 



Relations. By its superficial, or inferior surface, w r ith the platysma 

 myoides, the digastricus, the supra-hyoidean fascia, the submaxillary gland 

 and the submental artery. By its deep or superior surface, with the genio- 

 hyoideus, the genio-hyo-glossus, the stylo-glossus, the gustatory nerve, the 

 hypoglossal nerve, Wharton's duct, the sublingual gland, and the mucous 

 membrane of the floor of the mouth. 



After the mylo-hyoideus has been examined, it should be cut away from 

 its origin and insertion, and completely removed. The view of the next 

 muscles would also be greatly improved by dividing the lower jaw on the 

 near side of the symphysis, and drawing it outwards, or by removing it 

 altogether, if the ram us have been already cut across in dissecting the in- 

 ternal pterygoid muscle. The tongue may then be drawn out of the mouth 

 by means of a hook. 



The GENIO-HYOIDEUS (ysvsov, the chin) arises from a small tubercle upon 

 the inner side of the symphysis of the lower jaw, and is inserted into the 

 upper part of the body of the os hyoides. It is a short and slender muscle, 

 very closely connected with the border of the following. 



Relations. By its superficial or inferior surface, with the mylo-hyoideus , 

 by the deep or superior surface, with the lower border of the genio-hyo- 

 glossus. 



The GENIO-HYO-GLOSSUS (yXufltfa, the tongue) is a triangular muscle, 

 narrow and pointed at its origin from the lower jaw, broad and fan-shaped 

 at its attachment to the tongue. It arises from a tubercle immediately 

 above that of the genio-hyoideus, and spreads out to be inserted into the 

 whole length of the tongue, from its base to the apex, and into the body 

 of the os hyoides. 



Relations. By its inner surface with its fellow of the opposite side. 

 By its outer surface with the mylo-hyoideus, the hyo-glossus, the stylo* 

 glossus, lingualis, the sublingual gland, the lingual artery, and the hypo- 

 glossal nerve. By its upper border with the mucous membrane of the floor 

 of the mouth, in the situation of the frasnum linguae ; and by the lower 

 border with the genio-hyoideus. 



Actions. The whole of this group of muscles acts upon the os hyoides 

 when the lower jaw is closed, and upon the lower jaw when the os hyoides 

 is drawn downwards, and fixed by the depressors of the os hyoides and 

 larynx. The genio-hyo-glossus is, moreover, a muscle of the tongue ; its 

 action upon that organ shall be considered with the next group. 



