MUSCLES OF MASTICATION 219 



4. Levator palpebrae superioris. — This slender, flat muscle is almost entirely 

 within the orbital cavity. It arises on the pterygoid crest, passes forward above the 

 rectus oculi superior and below the lacrimal gland, and terminates in a thin tendon 

 in the upper lid. 



Action. — To elevate the u]:)per lid. 



Blood-supply. — Ophthalmic artery. 



Nerve-supply. — Oculomotor nerve. 



MUSCLES OF MASTICATION 



The muscles of this group are six in number in the horse. They arise chiefly 

 from the upper jaw and the base of the cranium, and are all inserted into the man- 

 dible. 



1. Masseter. — This muscle extends from the zygomatic arch and facial crest 

 over the broad part of the mandibular ramus. It is semi-elliptical in outline. 



Origin. — By a strong tendon from the zygomatic arch and the facial crest. 



Insertion. — The outer surface of the broad part of the ramus of the mandible. 



Action. — Its action is to l)ring the jaws together. Acting singly, it also carries 

 the lower jaw toward the side of the contracting muscle. 



Structure. — The superficial face of the muscle in its upper part is covered by a 

 strong, glistening aponeurosis, and several tendinous intersections partially divide 

 the muscle into layers. The fibers of the sui)erficial layer take origin from the 

 malar and maxilla only, and diverge somewhat to their insertion close to the thick 

 ventral border of the lower jaw. The fibers of the deep laj'er arise from the entire 

 area of origin, and pass straight to the border of the mandible ; it will be noted that 

 a small part, near the temporo-maxillary joint, is not covered by the superficial layer. 

 The two layers are separa])le only al)ove and behind; elsewhere they are fused. 



Relations. — ^Superficially, the skin and ])anniculus, the ])arotid gland, the- 

 transverse facial and masseteric vessels, and the facial nerve; dcepl}', the ramus of 

 the mandible, the buccinator, depressor labii inferioris, and mylo-hyoideus muscles, 

 the superior l:)uccal glands, the buccinator nerve, and two large varicose veins which 

 join the facial vein at the anterior edge of the muscle. The facial vessels and parotid 

 duct run along the anterior edge of the muscle; the duct, however, bends forward 

 about the middle of the border and leaves the muscle. 



Blood-supply. — Transverse facial and masseteric arteries. 



Nerve-supply. — Alandiliiilar nerve. 



2. Temporalis, — This muscle occupies the temporal fossa. 



Origin. — The rough part of the temporal fossa and the crests which surround it. 



Insertion. — The eoronoid process of the mandible, which it envelops. 



Action. — Chiefly to raise the lower jaw, acting with the masseter and internal 

 pterygoid muscles. 



Structure. — The surface of the muscle is covered with a glistening aponeurosis, 

 and strong tendinous intersections are found in its substance. The inner edge of 

 the muscle is quite thin, but as the fibers converge toward the much smaller area 

 of insertion, the muscle l^ecomes nearly an inch thick. It fuses somewhat with the 

 masseter. 



Relations. — Superficially, the scutiform cartilage and anterior muscles of the 

 external ear and the orbital fat; deeply, the temporal fossa and the deep temporal 

 vessels and nerves. 



Blood-supply. — Superficial and deep temporal, and mastoid arteries. 



Nerve-supply. — Mandibular nerve. 



3. Pterygoideus intemus (s. medialis). — This muscle occupies a position on 

 the inner surface of the ramus of the mandible similar to that of the masseter on the 

 outside. 



