226 FASCLE AND MUSCLES OF THE HORSE 



row muscle, extending along the ventral and lateral aspects of the trachea from the 

 sternum to the angle of the jaw. It forms the ventral boundary of the jugular furrow. 



Origin. — The cariniform cartilage of the sternum. 



Insertion. — The posterior border of the lower jaw. 



Action. — Acting together, to flex the head and neck; acting singly, to incline 

 the head and neck to the side of the muscle contracting. 



Structure. — The two muscles are fused at their origin, which is fleshy. Near 

 the middle of the neck they separate, and, becoming thinner, each muscle passes 

 under the parotid gland and terminates in a flat tendon. 



Relations. — Superficially, the cervical panniculus; deeply, the sterno-thyro- 

 hyoideus and omo-hyoideus muscles. The upper edge of the muscle is related to 

 the jugular vein, which lies in the jugular furrow. The carotid artery, the vagus, 

 sympathetic, and recurrent nerves also lie along the upper edge at the root of the 

 neck. The tendon passes under the submaxillary vein and the parotid gland, hav- 

 ing the submaxillary gland and stylo-maxillaris muscle on its inner side. 



Blood-supply. — Carotid artery. 



Nerve-supply. — -Ventral branches of the s]:)inal accessory and first cervical 

 nerves. 



4. Sterno-thyro-hyoideus (Sterno-thyroideus et sterno-hyoideus). — This is a 

 long, slender, digastric muscle, applied to the ventral surface of the trachea and 

 its fellow of the opposite side. 



Origin. — The cariniform cartilage of the sternum. 



Insertion. — (1) The external surface of the thyroid cartilage of the larynx; 

 (2) the body of the hyoid ])one. 



Action. — To depress and retract the hyoid bone, the base of the tongue, and 

 the larynx, as in deglutition. It may also fix the hyoid bone wdien the depressors 

 of the tongue are acting, as in suction. 



Structure. — -The origin of the muscle is fleshy, and as far as the middle of the 

 neck it blends with its fellow. The common belly is then interrupted by a tendon, 

 or sometimes two tendons, from which arise three or four fleshy bands. The lateral 

 bands diverge to reach their insertion into the thyroid cartilage; wdiile the inner 

 bands, closely applied to each other and blending with the omo-hyoideus, pass 

 straight forward to reach the ventral surface of the hyoid bone. 



Relations. — At the root of the neck the common belly has the sterno-cephalicus 

 below, and the carotid arteries and recurrent nerves above. Further forward the 

 trachea becomes the upper relation, and near the head the skin and fascia, the low^er 

 one. 



Blood-supply .—Csirotid artery. 



Nerve-supply. — -Ventral branch of the first cervical nerve. 



5. Omo-hyoideus (Subscapulo-hyoideus). — This is a thin, ribbon-Hke muscle, 

 almost entirely fleshy, which crosses the trachea very obliquely under cover of the 

 mastoido-humeralis. 



Origin.^The subscapular fascia close to the shoulder joint. 



Insertion.— The body of the hyoid bone, in common with the hyoid branch of 

 the preceding muscle. 



Action. — To depress the hyoid bone. 



Structure. — The muscle is composetl of parallel fleshy fibers, except at its origin, 

 where it has a thin tendon. 



Relations. — In the first part of its course the muscle passes forward between 

 the supraspinatus, anterior deep pectoral, mastoido-humeralis (outwardly), and 

 the scalenus (inwardly). It is intimately adherent to the mastoido-humeralis. 

 In the middle of the neck it is related superficially to the mastoido-humeralis, 

 sterno-cephalicus, and the jugular vein; deeply, to the rectus capitis anterior major, 

 the carotid artery, the vagus, sympathetic, and recurrent nerves, the trachea, and,. 



