450 THE MUSCLES 



Action. — (1) The posterior belly draws upwards and l)ackwards the hyoid 

 bone, as in the elevation of the larynx in the second part of deglutition; (2) the 

 anterior belly, acting from above, draws upwards and forwards the hyoid bone; 

 and, acting in the opposite direction, (o) it assists in the depression of the lower 

 jaw and in opening the mouth. Although a comparatively weak muscle, it acts 

 with considerable power in this movement, for it is inserted at the end of the lever 

 of the second order formed by the mandible, while the resistance which it has to 

 overcome is exerted by muscles acting much nearer to the fulcrum. Moreover, its 

 direction downwards and backwards is at a considerable angle with the line of the 

 lever, viz. that which joins the i)rominence of the chin to a point a little above 

 the inferior dental foramen. (4) If the mandible be fixed and both bellies act, the 

 hyoid bone will be drawn directly upwards. By this action the muscle is of great 

 importance in the elevation of the tongue, which rests upon the ui)per surface of 

 the hyoid bone. It Avill therefore help in the first part of deglutition, in which the 

 back of the tongue is pressed against the hard palate. 



Relations. — The posterior belly lies beneath the mastoid process, the sterno- 

 mastoid, splenius, and trachelo-mastoid muscles, the facial vein, and the parotid 

 gland; in front lies the stylo-hyoid muscle; deeply, the middle constrictor of the 

 pharynx, the hyo-glossus, the external and internal carotid arteries, and some of 

 the branches of the external carotid, the internal jugular vein, the hypoglossal and 

 superior laryngeal nerves. 



The tendon lies beneath the deep cervical fascia, platysma myoides, and part of 

 the stylo-hyoid muscle. Above is the submaxillar}- gland. On its deep surface is 

 the rest of the stylo-hyoid muscle, the mylo-hyoid, the hyo-glossus, and the hypo- 

 glossal nerve. 



The anterior belly is covered by the deep cervical fascia and platysma myoides, 

 and it lies upon the mylo-hyoid muscle. 



Variations. — A second posterior belly may arise in front of the angle of the mandible; slips 

 may also arise from the styloid process, or the pharynx. The anterior belly may be absent ; it 

 may be partly or entirely united with that of the opposite side, or may send a slip to the median 

 raphe of the mylo-hyoid. 



2. STYLO-HYOID 



The stylo-hyoid — named from its attachments — is a slender fusiform muscle 

 with a bifurcated lower extremity. 



Origin. — The back and outer surface of the styloid process of the temporal bone 

 near its base. 



Insertion. — The lower border of the body of the hyoid bone at the point of 

 union with the great cornu. 



Structure. — Arising by a short tendon, its fibres soon become fleshy and pass 

 downwards and forwards. Just before its insertion they divide into two bundles, 

 which form an obliquely directed arch bridging over the tendon of the digastric 

 muscle. 



Nerve -supply. — A special branch of the facial nerve, Avhich enters its deep 

 surface fr(nn Ijchind. 



Action. — The same as that of the posterior belly of the digastric, viz. to draw 

 the liyoid l)one backwards and upwards. 



Relations. — Superficially, the parotid gland and deep cervical fascia; in front, 

 the submaxillary gland; behind, the posterior belly of the digastric; deeply, the 

 middle constrictor and hyo-glossus and the external carotid artery. 



Variations, — The stylo-hyoid may arise in part from the cartilage of the external auditory 

 meatus. It may be absent, or its insertion may be undivided, in which case it may pass to the 

 inner or outer side of the digastric tendon. A second stylo-hyoid may run beneath the hyo-glossus 

 to the lesser cornu of the hyoid bone. 



3. MYLO-HYOID 



The mylo-hyoid — named from its attachment to the lower jaw (jj-uXtj = a mill 

 and the jaw) and to the hyoid bone — is a triangular and somewhat curved sheet. 



