268 FASCLE AND MUSCLES OF THE HORSE 
sympathetic, and recurrent nerves also lie along the upper edge at the root of the 
neck. The tendon passes under the external maxillary vein and the parotid gland, 
having the mandibular gland and occipito-mandibularis muscle on its medial side. 
Blood-supply.—Carotid artery. 
Nerve-supply.—Ventral branch of the spinal accessory nerve. 
4. Sterno-thyro-hyoideus (Sterno-thyreoideus 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) A prominence on the posterior border of the lamina of the thy- 
roid cartilage of the larynx at the ventral end of the oblique line; (2) the body 
and lingual process of the hyoid bone. 
Action.—To retract and depress the hyoid bone, the base of the tongue, and 
the larynx, as in deglutition. It may also fix the hyoid bone when the depressors 
of the tongue are acting, as in sucking. 
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 lat- 
eral small bands (Sterno-thyroidei) diverge to reach their insertion into the thy- 
roid cartilage by a delicate tendon; while the medial and larger bands (Sterno- 
hyoidei), 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 is related ventrally to 
the sterno-cephalicus, and the carotid arteries and recurrent nerves dorsally. 
Further forward the trachea becomes the dorsal relation, and near the head the 
omo-hyoideus, skin and fascia, the ventral one. 
Blood-supply.—Carotid artery. 
Nerve-supply.—Ventral branches of the first and second cervical nerves. 
5. Omo-hyoideus.'—This is a thin, ribben-like muscle, almost entirely fleshy, 
which crosses the trachea very obliquely. 
Origin.—The subseapular fascia close to the shoulder joint. 
Insertion.—The body and adjacent part of the lingual process of the hyoid 
bone, in common with the hyoid branch of the preceding muscle. 
Action.—To retract the hyoid bone and the root of the tongue. 
Structure—The muscle is composed of parallel fleshy fibers, except at its origin, 
where it has a thin tendon. 
Relations —The posterior part of the muscle is related laterally to the supra- 
spinatus, anterior deep pectoral, and brachiocephalicus and the prescapular lymph- 
glands; and medially to the scalenus. It is intimately adherent to the brachio- 
cephalicus. In the middle of the neck it is related superficially to the brachio- 
cephalicus, sterno-cephalicus, and the jugular vein; deeply, to the rectus capitis 
ventralis major, the carotid artery, the vagus, sympathetic, and recurrent nerves, 
the trachea, and, on the left side, the cesophagus. In its anterior part the muscle 
blends with the sterno-hyoideus, the two covering the sterno-thyroideus, the thy- 
roid gland in part, and the ventral face of the larynx. 
Blood-supply.—Carotid and inferior cervical arteries. 
Nerve-supply.—Ventral branch of the first cervical nerve. 
6. Scalenus (M. scalenus prime costse).—This muscle is deeply situated on 
the side of the posterior half of the neck. It is composed of two parts, between 
which the cervical roots of the brachial plexus of nerves emerge. 
Origin.—The anterior border and lateral surface of the first rib. 
Insertion —(1) The dorsal (smaller) part (M. sealenus dorsalis) is attached to 
the transverse process of the seventh cervical vertebra; (2) the ventral part (M. 
1 Also termed the subscapulo-hyoideus. 
