780 ORGANS OF DIGESTION. 



mucous membrane of the lips, around the opening of the mouth. They are 

 situated between the mucous membrane and the orbicularis oris muscle. 

 They are compound glands of a rounded form, the largest of them not ex- 

 cseding the size of a split pea ; and they open into the mouth by distinct 

 orifices. 



Between the buccinator muscle and the mucous membrane of the cheek, 

 by which it is lined in its whole extent, are the buccal glands, similar to the 

 labial glands, but smaller. Two or three glands, larger than the rest, found 

 between the masseter and buccinator muscles, and opening by separate ducts 

 near the last molar tooth, are called the molar glands. The duct of the 

 parotid gland also opens upon the inner surface of the cheek, opposite to the 

 second upper molar tooth. 



Immediately within the lips and cheek, are the dental arches, consisting 

 of the teeth, gums, and maxillse. The jaw-bones, the articulation and move- 

 ments of the lower maxilla, and the muscles used in mastication, are else- 

 where described. The gums (gingivse) are composed of a dense fibrous 

 tissue, connected very closely with the periosteum of the alveolar processes, 

 and covered by a red and highly vascular but not very sensitive mucous 

 membrane, which is smooth in its general surface, but is beset with fine 

 papillae in the immediate vicinity of the teeth. 



THE TEETH. 



In the human subject, as in mammalia generally, two sets of teeth make 

 their appearance in the course of life, of which the first constitutes the tem- 

 porary, deciduous, or milk teeth, whilst the second is named the permanent 

 set. The temporary teeth are twenty in number, ten in each jaw, and the 

 permanent set consists of thirty two, sixteen above and sixteen below. 



Deficiencies in the number of the teeth sometimes occur, and the number is frequently 

 increased by one or more supernumerary teeth. These are usually small, and pro- 

 vided with only a single fang ; and, though generally distinct, they are sometimes 

 attached to other teeth : they occur more frequently near the front than the hinder 

 teeth, and are more often met with in the upper than in the lower jaw. 



General Characters of the Teeth. Every tooth consists of three portions, 

 viz., one which projects above the gums and is named the body or crovin, 

 another which is lodged in the alveolus or socket, and constitutes the root 

 or fang, and a third, intermediate between the other two, and, from being 

 more or less constricted, named the cervix or neck. The size and form of 

 each of these parts vary in the different kinds of teeth. 



The roots of all the teeth are accurately fitted to the alveoli of the jaws, 

 in which they are implanted. Each alveolus is lined by the periosteum, which 

 also invests the contained fang as high as the cervix. This dental perios- 

 teum, sometimes named the periodontal membrane, is blended with the 

 dense and slightly sensitive tissue of the gums, which closely surrounds the 

 neck of the tooth. The roots of all the teeth taper from the cervix to the 

 point, and this form, together with the accurate adjustment to the alveolus, 

 has the effect of distributing the pressure during use over the whole socket, 

 and of preventing its undue action on the apex of the fang through which 

 the blood-vessels and nerves enter. 



The thirty-two permanent teeth consist of four incisors, two canines, four 

 bicuspids, and six molars in each jaw. The twenty temporary teeth are 

 four incisors, two canines, and four molars above and below. There are 



