264 FASCIiE 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. — Superficiallj^ 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 occipito-mandibularis. It passes downward and forward, 

 and is succeeded bj^ a small rounded tendon. The latter perforates the tendon of 

 insertion of the stjdo-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 oc- 

 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 (IMm. ossis hj'oidei), 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 raphe extending from the symphysis to the 

 hyoid bone; (2) the lingual process, body, and thyroid cornu of the hj^oid 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 raphe. It is 

 chieflj' 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, 

 hj'O-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 mandibulae. 



