336 MUSCLES AND FASCIAE. 



arises from the upper border of the scapula, and occasionally from the trans- 

 verse ligament which crosses the suprascapular notch ; its extent of attachment 

 to the scapula, varying from a few lines to an inch. From this origin, the 

 posterior beliy forms a flat, narrow fasciculus, which inclines forwards across 

 the lower part of the neck ; behind the Sterno-mastoid muscle, where it becomes 

 tendinous, it changes its direction, forming an obtuse angle, and ascends almost 

 vertically upwards, close to the outer border of the Sterno-hyoid, to be inserted 

 into the lower border of the body of the os hyoides, just external to the inser- 

 tion of the Sterno-hyoid. The tendon of this muscle, which varies much in its 

 length and form in different subjects, is held in its position by a process of the 

 deep cervical fascia, which includes it in a sheath, and is prolonged down to be 

 attached to the cartilage of the first rib. It is by this means that the angular 

 form of the muscle is maintained. 



This muscle subdivides each of the two large triangles at the side of the neck 

 into two smaller triangles; the two posterior ones being the posterior superior or 

 suboccipital, and the posterior inferior or subclavian; the two anterior, the anterior 

 superior or superior carotid, and the anterior inferior or inferior carotid triangle. 



Relations. By its superficial surface, with the Trapezius, Subclavius, the cla- 

 vicle, the Sterno-mastoid, deep cervical fascia, Platysma, and integument. By 

 its deep surface, with the Scaleni, brachial plexus, sheath of the common carotid 

 artery, and internal jugular vein, the descendens noni nerve, Sterno-thyroid and 

 Thyro-hyoid muscles. 



Nerves. The Thyro-hyoid is supplied by the hypoglossal; the other muscles 

 of this group by branches from the loop of communication between the descen- 

 dens and communicans noni. 



Actions. These muscles depress the larynx and hyoid bone, after they have 

 been drawn up with the pharynx in the act of deglutition. The Omo-hyoid 

 muscles not only depress the hyoid bone, but carry it backwards, and to one or 

 the other side. They are also tensors of the cervical fascia. The Thyro-hyoid 

 may act as an elevator of the thyroid cartilage, when the hyoid bone ascends, 

 drawing upwards the thyroid cartilage behind the os hyoides. 



3. SUPRA-HYOID EEGION. (Figs. 220, 221.) 

 ELEVATORS OF THE Os HYOIDES DEPRESSORS OF THE LOWER JAW. 



Digastric. Mylo-hyoid. 



Stylo-hyoid. Genio-hyoid. . 



Dissection. To dissect these muscles, a block should be placed beneath the back of the neck, 

 and the head drawn backwards, and retained in that position. On the removal of the deep 

 fascia, the muscles are at once exposed. 



The Digastric consists of two fleshy bellies united by an intermediate rounded 

 tendon. It is a small muscle, situated below the side of the body of the lower 

 jaw, and extending, in a curved form, from the side of the head to the sym- 

 physis of the jaw. The posterior belly, longer than the anterior, arises from the 

 digastric groove on the inner side of the mastoid process of the temporal bone, 

 and passes downwards, forwards, and inwards. The anterior belly, being reflected 

 upwards and forwards, is inserted into a depression on the inner side of the 

 lower border of the jaw, close to the symphysis. The tendon of the muscle 

 perforates the Stylo-hyoid, and is held in connection with the side of the body 

 of the hyoid bone by an aponeurotic loop, lined by a synovial membrane. A 

 broad aponeurotic layer is given off from the tendon of the Digastric on each 

 side, which is attached to the body and great cornu of the hyoid bone: this is 

 termed the supra-hyoid aponeurosis. It forms a strong layer of fascia between 

 the anterior portion of the two muscles, and a firm investment for the other 

 muscles of the supra-hyoid region which lie beneath it. 



The Digastric muscle divides the anterior superior triangle of the neck into 

 two smaller triangles; the upper, or submaxillary, being bounded, above, by 



