DIGESTIVE SYSTEM. 235 



of that part of the head which corresponds to the face in the hu- 

 man subject : and this development of feature is in none more 

 striking than in the horse and dog. Consequently, in these ani- 

 mals, the nose and mouth are cavities of large dimensions. And 

 in the horse, the mouth appears to have been thus prolonged, not 

 only to enable him to collect his food with more facility, but also 

 that he might subject greater parcels of it at a time to the action 

 of the grinding teeth, whereby the processes of mastication and 

 deglutition are greatly accelerated. 



Conformation. — The mouth is constructed, in part, of bone, and 

 in part of soft materials. The superior and anterior maxillary and 

 the palate bones form the roof; the inferior maxilla, the lower 

 part ; the incisive teeth, the front ; and the molar teeth, the sides. 

 The lips, cheeks, soft palate, gums, and buccal membrane, consti- 

 tute its soft parts. The tongue occupies its cavity, and the sali- 

 vary glands are appendages to it. 



Lips. 



General Conformation. — The lips, two in number, superior 

 and inferior, are attached to the alveolar projections of the supe- 

 rior and inferior maxillae, by the muscles that move them ; by 

 the cellular tissue entering into their composition ; and by the 

 membrane that lines them. Their borders surround and bound 

 the orifice of the mouth, and are united together on either side; 

 which points of union are denominated their commissures, or the 

 angles or corners of the mouth. Exteriorly, the lips are creased 

 down the middle by perpendicular lines of division ; exhibit little 

 papillary eminences upon their surface ; and present a softer and 

 shorter coating of hair than what is found in ordinary places, out 

 of which project several long straggling horse-hairs or whiskers. 

 The inferior lip is altogether smaller, and is thinner in substance 

 than the superior; and is distinguished by a remarkable promi- 

 nence about its centre, from which grows a tuft of long coarse 

 hairs, vulgarly designated as the beard. 



Structure. — The lips are both muscular and glandular in their 

 composition. Several small muscles (which have already come 

 under our observation*), arising from the maxillary bones, are 

 inserted into them, and endow them with great self-mobility: 

 one alone, consisting of circular fibres, is interwoven in their sub- 

 stance without having any other connexion ; this is denominated 

 the orbicularis oris, or sphincter labiorum, from its use, which is 

 that of closing the mouth. This muscle is an antagonist to all 

 the others ; they raise or depress the lips, or draw them to one 



* Vide " Anterior Maxillary Rej^ion," p. 8.9. 



