564 



NECK. 



internally joins a simi'ar process from the op- 

 posite side to form with it a tendinous expan- 

 sion, (often assisted by a few fleshy fibres from 

 the anterior belly of the digastric,) which reaches 

 from the os hyoides as far as the jaw, and con- 

 tributes to support the floor of the mouth. 



The relations of this muscle are complicated 

 and important : the convexity of its curve is 

 the upper limit of the anterior triangle of the 

 neck ; its concavity bounds a space, the area 

 of which extends within the jaw to the myloid 

 ridge, containing various parts, and named 

 from the muscle the digastric space; its pos- 

 terior belly crosses the external and internal 

 carotid, the facial, lingual, and occipital branches 

 of the former, the internal jugular vein, the 

 three divisions cf the eighth, the ninth, and the 

 sympathetic nerve, the side of the pharynx, the 

 trachelo-mastoid and styloid muscles, and the 

 hyo-glossus. The sterno-mastoid and splenius 

 cover its origin ; the portio dura emerges at its 

 anterior edge, aiong which the posterior aural 

 artery runs, and round which the posterior part 

 of the parotid gland is folded. Its anterior 

 belly and tendon support the subm<axilla 1 y 

 gland, are covered by fascia and platysma, and 

 correspond to the mylo-hyoid muscle, which 

 is covered and strengthened by the aponeurotic 

 expansion derived from the digastric. 



The action of this muscle varies according to 

 the fixity of the jaw: when the mouth is firmly 

 closed, the contraction of the two bellies will 

 draw the hyoid bone vertically upward, and 

 communicate to the pharynx the movement 

 of elevation, which adapts it for receiving the 

 masticated food. A firm closure of the jaw, a 

 contraction of the digastric muscles, and conse- 

 quent shortening of the pharynx, (indicated by 

 rising of the pomum Adami,) are well known 

 acts in the process of deglutition. When the 

 hyoid bone is fixed by its depressors (and 

 perhaps in some degree retracted by the joint 

 actions of the posterior belly of the digastric 

 and of the omo-hyoid), the anterior belly, both 

 passively as a reflected cord, and actively, 

 in virtue of its muscular fibres, depresses the 

 lower jaw and opens the mouth. Simulta- 

 neously, too, with its act of raising the pharynx, 

 this muscle must tighten, by its posterior belly, 

 the mesial aponeurotic expansion, which joins 

 it to its fellow; and, by so doing, must assist 

 the mylo-hyoid in raising the floor and reducing 

 the capacity of the mouth. It fulfils, there- 

 fore, important uses in the mechanism of de- 

 glutition. 



The stylo-hyoid muscle is an accessory to the 

 posterior belly of the digastric, and arises from 

 the outer surface of the styloid process, about 

 midway from its base, by a small round tendon, 

 which soon swells into an elongated body. 

 This lies along the posterior belly of the digas- 

 tric, parallel to its anterior edge, and, when it 

 reaches the os hyoides, is inserted into the outer 

 surface of that bone, at the union of its body 

 and cornu, by short aponeurotic fibres. It 

 usually divides, just previously to its insertion, 

 to give passage to the tendon of the digastric. 

 The portio dura of the seventh pair emerges 

 between its origin and that of the digastric : in 



other respects its relations so entirely agree with 

 those of the descending belly of that muscle, as 

 do likewise its uses, that no particular descrip- 

 tion of these is necessary. 



The mylo-hyoid muscles are so mutually de- 

 pendent that they might almost be described 

 as a single muscle. They arise on either side 

 from the oblique or myloid ridge on the buccal 

 surface of the lower jaw in its whole extent, 

 i. e. from opposite the last molar tooth to the 

 neighbourhood of the symphysis. The fleshy 

 fibres, that succeed the short aponeurosis of 

 origin, proceed parallelly toward the median 

 line, and are inserted into a raphe, which reaches 

 from the symphysis of the jaw to the body of 

 the hyoid bone, and likewise into the upper 

 border of the body of that bone. The anterior 

 fibres are short ; those which succeed progres- 

 sively increase in length, and the posterior, 

 which are fixed to the hyoid bone, are of all the 

 longest. Each muscle is, therefore, triangular, 

 having an outer edge by which it rises from 

 the jaw, an inner edge of union with its fellow, 

 and a posterior edge, which is seen to extend, 

 in the digastric space, from the posterior extre- 

 mity of the myloid ridge to the upper edge of 

 the body of the hyoid bone, close to its cornu. 

 The under surface of the muscle corresponds to 

 the submaxillary gland and to the insertion of 

 the digastric; its upper surface sustains the 

 tongue and floor of the mouth, from the mu- 

 cous membrane of which it is separated by 

 Wharton's duct, the sub-lingual gland, and gus- 

 tatory nerve; it also corresponds to the hyo- 

 glossus, genio-hyoideus, and genio-hyo-glossus, 

 and to the termination of the lingual artery 

 and nerve. The duct of the sub-maxillary 

 gland winds round its posterior edge, in pro- 

 ceeding to open beside the frsenum linguse. 

 The habitual state of this muscle is one in 

 which it is rendered, with its fellow, convex 

 downward by pressure of the superincumbent 

 parts; and no its surfaces cannot strictly be 

 said to face upward and downward, but with 

 a modification of these directions respectively 

 inward and outward. Thus the two muscles 

 furnish a concave floor to the mouth, and it is 

 only in their contraction, which accordingly 

 diminishes the cavity, that this becomes strictly 

 horizontal. Their action, especially when as- 

 sisted by other muscles, is to propel the mas- 

 ticated food by lessening the capacity of the 

 mouth. 



The hyo-glossus is a thin quadrilateral plane 

 of parallel muscular fibres, having the attach- 

 ments which its name indicates. It rises from 

 the entire length of the great cornu and ad- 

 joining part of the body of the os hyoides, on 

 their upper surface, and ascends to be inserted 

 into the side of the tongue. From beneath its 

 anterior thicker edge the lingual artery emerges; 

 its posterior thin border receives the insertion 

 of the stylo-glossus ; its deep surface corres- 

 ponds to the genio-hyo-glossus and lingualis, 

 from the former of which it is partly separated 

 by the lingual artery; its external face is sepa- 

 rated from the mylo-hyoid muscle by the lin- 

 gual and gustatory nerves and duct of the sub- 

 maxillary gland. 



