THE SUPRA- AND INFRAHYOID MUSCLES 



391 



border of the sternum. The boundaries of the posterior triangle are, in front, the posterior border 

 of the Sternocleidomastoideus; below, the middle third of the clavicle; behind, the anterior margin 

 of the Trapezius. The apex corresponds with the meeting of the Sternocleidomastoideus and 

 Trapezius on the occipital bone. The anatomy of these triangles will be more fully described 

 with that of the vessels of the neck (p. 562). 



Nerves. The Sternocleidomastoideus is supplied by the accessory nerve and branches from 

 the anterior divisions of the second and third cervical nerves. 



Actions. When only one Sternocleidomastoideus acts, it draws the head toward the shoulder 

 of the same side, assisted by the Splenius and the Obliquus capitis inferior of the opposite side. 

 At the same time it rotates the head so as to carry the face toward the opposite side. Acting 

 together from their sternoclavicular attachments the muscles will flex the cervical part of the 

 vertebral column. If the head be fixed, the two muscles assist in elevating the thorax in forced 

 inspiration. 



Fia. 385. Muscles of the neck. Lateral view. 



m. THE SUPRA- AND INFRAHYOID MUSCLES (Figs. 385, 386). 



The suprahyoid muscles are : 



Digastricus. 

 Stylohyoideus. 



Mylohyoideus. 

 Geniohyoideus. 



The Digastricus (Digastric muscle} consists of two fleshy bellies united by an 

 intermediate rounded tendon. It lies below the body of the mandible, and extends, 

 in a curved form, from the mastoid process to the symphysis menti. The posterior 

 belly, longer than the anterior, arises from the mastoid notch of the temporal 

 bone and passes downward and forward. The anterior belly arises from a depression 

 on the inner side of the lower border of the mandible, close to the symphysis, and 

 passes downward and backward. The two bellies end in an intermediate tendon 

 which perforates the Stylohyoideus muscle, and is held in connection with the side 

 of the body and the greater cornu of the hyoid bone by a fibrous loop, which is 



