COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 141 



g) Digastric. The muscle along the ventral surface of each half of the 

 jaw bone, terminating in a slender tendon. Origin, occipital bone; insertion, 

 ventral surface of the mandible; action, opens the jaw (depressor of the 

 jaw). 



h) Mylohyoid. The thin sheet of muscle crossing transversely between 

 and dorsal to the two digastrics. Origin, mandible; insertion, median ventral 

 line (raphe) and the hyoid; action, raises the floor of the mouth and brings 

 the hyoid forward. 



i} Sternothyroid. Divide the two sternohyoids in the median ventral line. 

 This exposes the trachea or windpipe, a tube stiffened by rings of cartilage. At 

 the top of the trachea at a level about between the two submaxillary glands 

 there is an enlarged chamber, the larynx or Adam's apple, whose walls are sup- 

 ported by cartilage. The chief cartilage of the larnyx is the large shield-shaped 

 cartilage, which forms the ventral wall; this is called the thyroid cartilage. The 

 Sternothyroid muscle will be found, one on each side of the trachea, dorsal to 

 the sternohyoid, originating on the sternum and inserted on the thyroid cartilage. 

 Action, pulls the larynx posteriorly. 



j} Thyrohyoid. A thin muscle at each side of the larynx, extending from 

 the thyroid cartilage to the hyoid. Action, raises the larynx. 

 Cat: 



a) Sternomastoid. This is the superficial muscle of the ventral side of 

 the neck. A large vein, the external jugular vein, crosses its surface at an angle 

 to the direction of its fibers. Origin by two parts, from the median raphe and 

 themanubrium of the sternum, the first-named origin lying ventral to the second, 

 so that the muscle appears divisible into two muscles. From the origins the 

 muscle passes obliquely away from the median ventral line around the sides of 

 the neck and is inserted on the skull from the lambdoidal ridge onto the mastoid 

 process. The muscle passes internal to the submaxillary gland and the parotid 

 gland ; the latter is a mass at the base of the ear. The insertion on the mastoid 

 process is by means of a thick tendon. Action, singly turn the head, together 

 depress head on neck. 



b} Sternohyoid. The anterior ends of these muscles are visible between 

 the two sternomastoids, as the latter diverge from the median raphe. Slit the 

 raphe of the sternomastoids to the manubrium of the sternum, thus exposing 

 the full length of the sternohyoids. They extend in the median ventral line 

 from the first costal cartilage to the body of the hyoid bone, the two being closely 

 united in the median line. Action, draw the hyoid posteriorly. 



c) Cleidomastoid. Lateral to the sternomastoid is a long muscle passing 

 from the head to the upper arm. Loosen this up and find internal to it a narrow 

 flat muscle, the cleidomastoid. Origin, clavicle, which will be found as a 

 slender bone on the internal surface of the long muscle just mentioned at the 

 level of the shoulder; insertion, mastoid process, dorsal to the insertion of the 



