264 FASCLE AND MUSCLES OF THE HORSE 
Structure.—The muscle contains a good deal of tendinous tissue. It blends 
with the posterior belly of the digastricus. 
Relations.—Superficially, the parotid gland, the tendon of the sterno-cephalicus, 
and the fibrous expansion which connects it with the tendon of the brachiocephali- 
cus; deeply, the guttural pouch, the external carotid artery, the ninth and twelfth 
nerves, the pharynx, and the mandibular salivary gland. 
Blood-supply.—External carotid artery. 
Nerve-supply.—Facial nerve. 
6. Digastricus.—This muscle is composed of two fusiform, flattened bellies, 
united by a round tendon. 
Origin.—The paramastoid process of the occipital bone, in common with the 
preceding muscle. 
Insertion —The medial surface of the ventral border of the molar part of the 
ramus of the mandible. 
Action.—It assists in depressing the lower jaw and opening the mouth. If the 
mandible be fixed and both bellies contract, the hyoid bone and the base of the 
tongue are raised, as in the first phase of deglutition. 
Structure-—The posterior belly has the appearance of a branch detached from 
the medial surface of the oecipito-mandibularis. It passes downward and forward, 
and is sueceeded by a small rounded tendon. The latter perforates the tendon of 
insertion of the stylo-hyoideus, and is provided with a synovial sheath. The an- 
terior belly is larger and terminates by thin, tendinous bundles. 
Relations—The posterior belly has practically the same relations as the oe- 
cipito-mandibularis. The intermediate tendon is in contact laterally with the me- 
dial pterygoid muscle, the mandibular gland and duct, and the external maxillary 
artery. The anterior belly lies in the mandibular space between the ramus of the 
jaw and the mylo-hyoideus muscle; the sublingual vessels run along its dorsal 
border. 
Blood-supply.— External carotid and sublingual arteries. 
Nerve-supply.— Facial and mandibular nerves. 
THE HYOID MUSCLES 
This group consists of eight muscles (Mm. ossis hyoidei), one of which, the 
hyoideus transversus, is unpaired. 
1. Mylo-hyoideus.'.—This muscle, together with its fellow, forms a sort of 
sling between the rami of the mandible, in which the tongue is supported. 
Origin.—The medial surface of the alveolar border of the mandible. 
Insertion.—(1) A median fibrous raphé extending from the symphysis to the 
hyoid bone; (2) the lingual process, body,.and thyroid cornu of the hyoid bone. 
Action.—It raises the floor of the mouth, the tongue, and the hyoid bone. 
Structure—Each muscle consists of a thin curved sheet, the fibers passing 
ventrally from their origin and then curving toward the median raphé. It is 
chiefly fleshy, and is thickest behind. There is a tendinous intersection between this 
muscle and the omo-hyoideus, to which both muscles are attached. The anterior 
superficial part of the muscle is termed the mylo-glossus. 
Relations.—On the superficial surface of the muscles are the ramus, the medial 
pterygoid and digastricus muscles, and the mandibular lymph-glands. The deep 
surface is in contact with the mucous membrane of the mouth, the stylo-glossus, 
hyo-glossus, and genio-hyoideus muscles, the sublingual gland and vessels, the 
mandibular duct, and the lingual and hypoglossal nerves. The sublingual vein 
passes through the posterior part. 
Blood-supply.—Sublingual artery. 
1 Also known as the transversus mandibulz. 
