HEAD AND NECK OF THE HORSE 79 



(m. orbicularis oris) and the terminations of such other muscles that 

 converge upon the oral fissure. The orbicular, zygomatic, naso-labial 

 and canine muscles, the depressor of the lower lip and the levator of 

 the upper lip have been examined along with other constituents of the 

 face. There remain the incisive and mental muscles, and these may be 

 dissected after the mucous membrane has been examined. (3) Small 

 collections of labial glands — better developed in the upper lip and 

 more numerous towards the commissures — form an imperfect third 

 layer. They are in the shape of small yellowish masses that elevate the 

 mucous membrane and can be seen shining through it. (4) The 

 mucous membrane lining the inner surface of each lip is relatively thin 

 and continuous with that clothing the rest of the cavity of the mouth. 

 A frcenulunn of the lip (frsenulum labii), such as is conspicuous in 

 man, is absent in the horse. Skin and mucous membrane meet at the 

 free border of the lip. 



Min. incisivi. — In order to expose the incisive muscles, the mucous 

 membrane must be dissected away from the inner surface of the lips. 

 The upper incisive muscle (m. incisivus superior) arises from the 

 alveolar border of the incisive bone from the second incisor to the 

 canine, or possibly a little beyond. Its fibres run into the lip and the 

 lateral wing of the nostril. The lower incisive muscle (\w. incisivus 

 inferior) has a corresponding origin from the mandible and ends in the 

 lower lip and the chin. 



M. mentalis. — This forms the muscular component of the chin. 

 Its fibres arise from the incisive part of the mandible and end, some in 

 the skin of the chin, and some by blending with the orbicular muscle in 

 the lower lip. 



Vessels and nerves of the lips. — The distribution of the superior 

 and inferior labial arteries has been noted previously. At the incisive 

 foramen an additional artery of moderate size should be looked for — 

 u. no.8olahialis— formed by the union of the end of the two greater 

 palatine arteries. After freeing itself of the incisive foramen, the vessel 

 divides into two branches that ramify in the upper lip and anastomose 

 with branches of the superior labial artery. 



The nerves ending in the lips are derived from the dorsal and ventral 

 buccal branches of the facial (motor) and the infraorbital and mental 

 branches of the trigeminal nerve (sensory). 



The tongue (Lingua). — The tongue is an organ concerned in 

 mastication, deglutition and taste, and consists mainly of a mass of 

 muscular tissue covered by mucous membrane. It is placed between the 



