378 STRUCTURE OF THE TEETH, 



the first pair of teeth in the alveolar cavity. It appears a soft 

 pulpy substance secreted by a membranous capsule, which in an 

 incisor tooth is single ; but in the molar there are no less than 

 four to the under and five to the upper, which accounts for the 

 irregular appearance of the molar teeth. The pulp is gradually 

 changed into the hard material. The membrane of the incisor 

 teeth that secreted the pulp is double ; and from its outer sur- 

 face it afterwards secretes the enamel, and from its inner the 

 ivory. A tooth is divided into the crown and the fang ; the for- 

 mer being that portion outside the gum, and the latter that con- 

 tained within the socket; whilst the part immediately embraced 

 by the gum is called the neck. The upper surface of the crown 

 is called the face, and is that part on which the mark is situated 

 in the incisor teeth, by which we judge of the age. 



The horse has two sets of teeth, tlie temporary and the per- 

 manent: the former are considerably smaller and whiter than 

 the latter. The tushes, however, are permanent, and do not 

 appear till after four years old. 



The permanent incisor teeth are too inches and upwards in 

 length : their fiice presents a deep funnel-like cavity, which ex- 

 tends some distance into the interior of tlie tooth, and consti- 

 tutes what is called the mark. The interior of the tooth is also 

 provided with a cavity that communicates with the bottom of the 

 socket by an opening at tlie end of the root. (See page 379., 

 fig. 12.) The tooth, therefore, when young, is nearly hollow; 

 but both cavities diminish with age. The lower one is filled 

 with a gelatinous substance, which contains the nerves and 

 vessels by which the tooth is nourished. The arrangement of 

 the ivory and the enamel in the incisor teeth deserves particular 

 consideration. " The enamel," says M. Girard, " after having 

 covered the entire of the external surface, doubles itself towards 

 the surface of wear, and dips down into the interior of the tooth, 

 forming a conical cavity which becomes narrow, and approaclies 

 towards the posterior edge of the tooth, the more according as it 

 is nearer the root. This prolongation of the enamel presents, 

 then, two portions for our consideration: — first, the cavity that 

 it forms to the exterior: secondly, the horn or infundibulum 

 which envelopes this cavity. This funnel, which is very easy to 

 perceive in making different sections of the tooth, is surrounded 

 in young teeth by the cavity of the pulp, which is continued to 

 the extremity of the free portion, but which is found more espe- 

 cially on the side corresponding to the anterior edge of the tooth.* 

 As the animal advances in age, this outer cavity becomes ob- 



* In many young teeth, on looking Into the lower cavity at the root, we 

 can see the bottouaof the funnel (see fig. 12.), wliieh forms the upper cavity, 

 inclining to the posteriui- part of the tooth ; the tooth, indeed, appears some- 

 what like a man's double nightcap, one part being folded within the other. 



