170 DISSECTION OF THE DOG 



The mouth cavity proper is contained within the dental arches, by which, 

 and the gums connected therewith, it is bounded in front and on each side. 

 Behind, it communicates with the cavity of the pharynx through the isthmus 

 faucium. The roof of the cavity is formed by the hard and soft palates, while 

 the floor is formed by the tongue and the mucous membrane extending there- 

 from to the mandible. Into this part of the mouth open the ducts of the 

 submaxillary and sublingual glands. 



Labia oris. — The two lips are not alike. In all dogs the upper Up is the 

 better developed and the more moveable, and overhangs the lower lip. In some 

 breeds the overlapping is considerable. The upper lip, moreover, is marked 

 in the middle line by a deep, vertical groove, the philtrum ; and in some breeds 

 there is, in addition, a notch of greater or lesser depth indenting its free edge 

 in the middle line. 



Behind the level of the canine tooth the free margin of the lower lip is 

 generally sharp and dentated. 



Each lip may be regarded as containing four layers : (1) The outer surface 

 is covered with skin provided with hair except over a small, triangular, median 

 area of the upper Up. The hairs are of two kinds. The most numerous are 

 variable in length — depending upon the breed — and relatively fine. Longer 

 and stronger tactile hairs are fairly abundant on the upper Up, but more scanty 

 on the lower ; (2) a muscular layer composes the greater part of the entire 

 thickness of each lip, and consists of a sphincter muscle (m. orbicularis oris) 

 and the terminations of such other muscles as converge upon the oral fissure ; 

 (3) smaU collections of labial glands constitute an imperfect third layer ; (4) the 

 inside of each Up is clothed by mucous membrane continuous with that lining 

 the rest of the mouth cavity. In the median plane the mucous membrane is 

 raised into a smaU fold, the frcenulum of the lip (frsenulum labn). Skin and 

 mucous membrane meet at the margin of the Ups. 



Buc&e. — The cheeks, which form the lateral boundary of the mouth cavity, 

 have the same general structure as the Ups. The glands (glandulse buccales) 

 are arranged in two groups — an upper and a lower. The former group is 

 considerable in volume and disposed below the orbit, where it wiU be examined 

 at a later period. The lower glands, less voluminous and more diffuse, extend 

 along the cheek in a line with the alveolar border of the mandible, and are 

 continuous with the glands of the lower Up. 



Each cheek is pierced by the parotid duct, which opens into the mouth on 

 the summit of a low elevation, the salivary papilla (papiUa salivalis), about the 

 level of the third maxillary cheek-tooth. The ducts of the buccal glands also 

 open on the surface of the mucous membrane. 



Dbntes. — A tooth consists of a crown (corona dentis) visible above the gum, 

 a neck (coUum dentis) embraced by the gum, and a root or roots (radix [radices] 

 dentis) embedded in the bony tooth cavity (alveolus dentaUs). The crown 

 of the tooth has a chewing surface (facies masticatoria), a surface covered 



