158 MYOLOGY, OR ANATOMY OF MUSCULAR SYSTEM 



a fascia invests the posterior belly of the omohyoid and holds 

 it down to the first rib, there connected with the costocoracoid 

 membrane. A process (pretracheal portion) also passes behind 

 the depressors of the hyoid, invests the thyroid body, passes 

 to the trachea, forms the carotid sheath, and extends to the 

 pericardium. Deepest of all is the prevertebral fascia. Inside 

 the pharyngeal muscles is the pharyngeal aponeurosis, outside 

 them their proper fascial layer (buccopharyngeal), connected 

 to the prevertebral fascia by areolar tissue, forming the rctro- 

 phan/ngeal space. A prolongation of the prevertebral fascia 

 forms the axillary sheath. 



2. M. Sternocleidomastoideus (its full name should mention 

 its insertion into the occipital bone). Origin, sternal head, 

 thick and round, from anterior surface of the manubrium; 

 clavicular, from the inner third of the upper surface of the 

 clavicle. The two portions meet, pass up and back, and insert 

 into the anterior border and outer surface of the mastoid 

 and outer half or more of the superior curved line of the 

 occiput, to meet the trapezius. Spinal accessory nerve pierces 

 the under surface of the external portion. 



Nerves. Both by the spinal accessory, offsets of which 

 are joined by the second cervical. 



Actions. The two sternomastoids draw the head and neck 

 forward toward the sternum; one, acting slightly, flexes the 

 head (extends, Henle) and flexes laterally and rotates, so 

 that the face looks up and toward the opposite side. Fixed 

 above, the muscles elevate the thorax in forced inspiration. 



3. Digastric muscle (M. biventer mandibulse) has two bellies 

 united by a rounded tendon; the posterior belly rises from 

 the digastric fossa on the mastoid portion of the temporal 

 bone, passes down, in, and forward toward the hyoid bone. 

 The anterior belly is attached close to the symphysis of the 

 lower jaw, on its inner surface close to the inferior margin, 

 and directed down, back, and slightly outward; the inter- 

 vening tendon is attached to the body and great cornu of 

 the hyoid by an aponeurosis and by the stylohyoid muscle, 

 which is pierced by the digastric tendon. The anterior bellies 

 of the two muscles are connected by a dense aponeurosis. 



Nerves. Anterior belly by the mylohyoid branch of the 

 inferior dental from the third division of the fifth nerve; pos- 

 terior belly by the facial. 



