332 DIGESTIVE SYSTEM OF THE HORSE 



(superioris, inferioris). The free border of the lip is dense and presents short, 

 very stiff hairs. The attached border is continuous with the surrounding struc- 

 tures, and is adherent to the alveolar borders of the bones of the jaws. 



Structure. — The lips are covered externally by the skin, and are lined by 

 mucous membrane; between these are muscular tissue and glands. The skin 

 lies directly on the muscles, many fibers of which are inserted into the former. The 

 muscles have been described (page 214). The labial glands (Glandulse labiales) 

 form a compact mass near the angles; they are numerous in the upper lip, fewer 

 in the lower. The mucous membrane is often pigmented, and is reflected upon 

 the bones of the jaws to form the gums. 



Blood-vessels and Nerves. — -The arteries are derived from the superior and 

 inferior labial and palato-labial arteries. The sensory nerves come from the 

 trigeminus, and the motor from the facial nerve. 



The cheeks (Buccse) form the sides of the mouth, and are continuous in front 

 with the lips. They are attached above and below to the alveolar borders of the 

 bones of the jaws. 



Structure. — This comprises: (1) The skin; (2) the muscular and glandular 

 layer; (3) the mucous membrane. The skin offers nothing in particular to notice. 

 The muscular tissue is formed mainly by the buccinator, ])ut also by parts of the 

 panniculus, zygomaticus, dilatator naris lateralis, levator nasolabialis, and depressor 

 labii inferioris. The buccal glands (Glandulse buccales) are arranged in two rows. 

 The upper row (superior buccal or molar glands) is found on the outer surface of 

 the buccinator muscle, near its upper border.^ The anterior part of the row con- 

 sists of scattered lobules; the posterior part, which lies under cover of the masseter 

 muscle, is more developed and compact. The lower row (inferior buccal or molar 

 glands), less voluminous than the upper, is situated in the submucous tissue at the 

 lower border of the buccinator muscle. The mucous membrane is reflected above 

 and below upon the gums, and is continuous behind with that of the soft palate. 

 It is reddish in color and frequently shows pigmented areas. Opposite the third 

 upper cheek tooth is the opening of the parotid (Stenson's) duct, surrounded by a 

 circular fold of the mucous membrane. A linear series of small papillae above 

 and below indicates the orifices of the small ducts from the buccal glands. 



Blood-vessels and Nerves. — The blood-supply is derived from the facial and 

 buccinator arteries. The sensory nerves come from the trigeminus and the motor 

 from the facial nerve. 



The gums (Gingivse) are composed of a dense fibrous tissue intimately united 

 with the periosteum of the alveolar processes, and blending at the edges of the alveoli 

 with the alveolar periosteum, which fixes the teeth in their cavities. They are 

 covered by a smooth mucous membrane, destitute of glands, and of a low degree 

 of sensibility. 



The hard palate (Palatum durum) is bounded in front and on the sides by 

 the alveolar arches; behind it is continuous with the soft palate. Its osseous basis 

 is formed by the premaxilla, maxilla, and palate bones. The mucous membrane 

 is smooth, and is attached to the bones by a submucosa which contains in its 

 anterior part an exceedingly rich venous plexus, constituting an erectile tissue. 

 A central raphe (Raphe palati) divides the surface into two equal portions. Each 

 of these presents about eighteen transverse curved ridges (Rugse palatini) which 

 have their concavity and their free edges directed backward. They are further 

 apart and more prominent anteriorly. There are no glands in the submucosa. 



Vessels and Nerves. — The blood-supply is derived chiefly from the palatine 

 arteries and the nerves from the trigeminus. 



The soft palate (Palatum moUe) is a musculo-membranous curtain which 

 separates the cavity of the mouth from that of the pharynx. It slopes downward 

 and Ijackward from its junction with the hard palate. The oral surface faces 



