18 4 THE ANATOMY OF THE HORSE. 



of the mouth. As will be seen at a later stage, after the removal 

 of the masseter, the superior buccal gland lies on the muscle at 

 its anterior edge, while the inferior buccal gland lies under its 

 posterior edge, and the mucous membrane of the mouth lines its 

 inner surface. 



Action. — When unopposed by the orbicularis oris the muscle will 

 retract (elevate) the angle Of the mouth ; but otherwise, as in 

 mastication, it compresses the cheek against the teeth and their alveoli, 

 and tends to keep the food between the upper and the lower molars. 

 In man, when the cheeks are distended, as in blowing a wind instru- 

 ment, the muscle compresses the volume of air and propels it as a 

 stream from the mouth ; hence the name, from the L. buccina, a 

 trumpet. 



The Depressor Labii Inferioris (Plate 29). This muscle is placed 

 along the hinder edge of the buccinator, with which it is confounded at 

 its origin from the alveolar edge of the inferior maxilla above the last 

 molar tooth. It terminates in the texture of the lower lip. 



Action.— To depress (retract) the lower lip. 



The Orbicularis Oris (Plate 29). This muscle surrounds the 

 aperture of the mouth like a sphincter. It cannot be described as 

 having either origin or insertion, its fibres forming a continuous ellipse. 

 In both lips the fibres are intimately adherent to the skin, and they are 

 partially blended with the labial insertions of some of the muscles 

 already dissected. 



Action. — To approximate the lips, as in the simple act of closing the 

 mouth or in prehension. 



Direction*. — Evert the upper lip, and wipe its exposed mucous 

 lining clean. Observe that it is studded with numerous short, 

 tubercle-like papillae. Each of these is perforated by the duct of 

 a labial mucous gland lying beneath the mucous membrane. Now 

 dissect away the mucous membrane so as to expose these glands. 

 At the same time, there will be brought into view the following 

 muscle : — 



The Depressor Labii Superioris. Under this name there is 

 described a bundle of muscular fibres that in the human subject is 

 reckoned a part of the buccinator. On each side the muscle arises 

 from the premaxillary bone above the corner incisor and the inter- 

 dental space as far as the canine tooth ; and, on the other hand, 

 its fibres terminate in the upper lip, blending with the orbicularis. 

 Branches of the infra-orbital and ' 7th nerves enter the lip between 

 the outer edge of this muscle and the lowest fibres of the buccinator. 



Action. — To assist the orbicularis by depressing the upper lip. 



The Palato-Labial Artery. While the upper lip is kept everted, 

 dissect backwards on the middle line until this artery is found coming 



