478 



HUMAN ANATOMY. 



tinuous with the intermediate tendon. This is bound down to the greater horn and 

 body of the hyoid bone by a puUey-hke band of the cervical fascia and to a certain 

 extent by the stylo-hyoid muscle, which divides near its insertion into the hyoid into 

 two slips, between which the tendon of the digastric passes. 



The posterior belly (Fig. 502) takes its origin from the mastoid groove of the 

 temporal bone, and passes downward and forward to become connected with the 

 intermediate tendon. 



Nerve-Supply. — The anterior belly is supplied by the mylo-hyoid nerve from 

 the inferior dental branch of the mandibular division of the trigeminus, the posterior 

 belly by the digastric branch of the facial. 



Action. — The digastric either raises the hyoid bone or depresses the jaw, 



Fig. 497. 



Mandible 



Mylo-hyoid 



Hyoid bone- 



Thyro-hyoid membrane 



Stylo-glossus- — - 

 Internal pterygoid 



Thyro-hyoid 

 Thyroid cartilage 

 Stylo-pharyngeus 



Crico-thyroid- 





Genio-hyoid 

 (mylo-hyoid removed) 



Digastric, 

 anterior belly 



\ Fascial loop binding 



-^ tendons to hyoid bone 



Stylo-glossus 



Stylo-hyoid 

 Masseter 



Digastric, 

 posterior belly 



..A'^ LThyroid gland 



Submental muscles from below ; trachea has tieen displaced downward and backward. 



according as one or other of the bones is fixed by the antagonizing muscles. By raising 

 the hyoid when the mandible is fixed, it assists the mylo-hyoid in pressing the tongue 

 against the palate during the first portion of the act of deglutition, and in the second 

 portion of that act the posterior belly will assist the stylo-hyoid in drawing the hyoid 

 upward and backward and so help in elevating the larynx. 



Relations. — The anterior belly rests upon the mylo-hyoid muscle. The pos- 

 terior belly is covered by the sterno-mastoid and splenius muscles, and crosses both 

 the external and internal carotid arteries, the internal jugular vein, and the pneumo- 

 gastric and spinal accessory nerves. 



Variations. — A close relationship exists between the mylo-hyoid and the anterior belly of 

 the digastric, and there is usually more or less exchangee of fibres between the two muscles, 

 sometimes amounting to a complete fusion. A duplicity of the anterior belly is a rather fre- 



