THE MOUTH. 423 



first than the second of these animals, and not so lono^. (Flower also observes 

 that the loop of enamel above the B is either absent, or scarcely perceptible in 

 the Asinine tribe.) 



7. The Mouth in General. 



We will now consider, as a whole, the cavity, the various parts of which have 

 been studied in detail ; and examine, successively, its general arranfjement, 

 capacitij, and mucous memhrane. 



Genei-al disposition and capacity of the mouth. — The mouth, being elongated 

 in the direction of the head, offers a great antero-posterior diameter, and two 

 small diameters — one vertical, the other transverse. The first extends from the 

 base of the epiglottis to the anterior opening of the mouth ; the second, from 

 the palate to the floor of the mouth ; and the third, from one jaw to the other. 

 AYhen the jaws are in contact, the space included between these limits is divided 

 into two regions : one central, the other peripheral. The first is circumscribed 

 by the dental arches ; the second is comprised between these arches on the one 

 side, and the cheeks and inner aspect of the lips on the other. It may therefore 

 be remarked, that the capacity of the mouth is almost null in these regions. The 

 cheeks and lips, in reality, lie almost exactly against the alveolar arches, and the 

 tongue, in contact with the palate by its superior surface, almost entirely fills 

 the central region. If the jaws separate from one another, and the cheeks recede 

 from the dental arches, the cavity of the mouth becomes enlarged in proportion 

 as these movements are extensive. It must be remembered that the separation 

 of the jaws is effected in an angular manner, and that the dilatation produced in 

 the mouth by this movement is greater before than behind, the opening of the 

 angle comprised between the two jaws being directed towards the entrance to 

 the cavity. 



Mucous memhrane. — The walls of the buccal cavity are covered by a tegu- 

 mentary membrane, which we have hitherto only examined in parts in the 

 different regions it covers ; but which, it is to be noted, forms here a single and 

 continuous layer — the mucous memhrane of the mouth. 



This membrane is continuous with the external skin around the margin of 

 the buccal opening ; from this point into the interior of the cavity, it extends at 

 first over the internal surface of the lips, then is prolonged backwards on the 

 cheeks as far as the posterior pillars of the tongue. If it is examined above and 

 below, to the bottom of the groove at the junction of the lips and cheeks, it is 

 seen to cover the maxillary bones and envelop the base of the teeth, where it 

 constitutes the gums. From the superior dental arch, it extends over the 

 palatine arch and the soft palate. And from the inferior arch, it descends to 

 the floor of the mouth, and is reflected over the tongue to form a covering for 

 that organ. At the isthmus of the fauces it is continuous with the pharyngeal 

 mucous membrane. 



The organization of the mucous membrane of the mouth is perfectly in 

 harmony with the digestive acts performed in that cavity. It is there where 

 the ahmentary substances, which are sometimes very hard, very resisting, and 

 covered with asperities, are crushed ; and to escape inevitable injury, this 

 membrane is protected by a very thick epithelium in those places which are 

 particularly exposed to the contact of these substances — such as the upper 

 surface of the tongue, the palate, and the cheeks ; even the coriimi, or sub- 

 epidermic layer, is also greatly thickened. But nature has not made this 



