292 



SPLANCHNOLOGY. 



A 



Fig. 95. 



Incisor aud canine teeth of a 



Horse. A, Front, B B, Lateral, 



and C C, Corner incisors ; D D, 



Canine teeth. 



axis following the line of the dental arch. As it Avears, it narrows 

 laterally, but its short axis widens, until in old age it is nearly 

 round, what was the fang being in wear. 

 The free surface of an incisor tooth, 

 excepting the table, is covered by a layer 

 of enamel ; the fang, which is a single 

 process, being covered by crusta petrosa. 

 Towards the centre of the table, in a 

 young tooth, a second ring of enamel is 

 visible, which is the mouth of a funnel- 

 shaped cavity called the irifundibuluTn. 

 This cavity in the young animal is ovoid, 

 its long axis following that of the table ; 

 it is lined by crusta petrosa, which, be- 

 coming stained by the food, constitutes 

 the so-called "mark." The space be- 

 tween the two tubes of enamel is filled 

 up with dentine; hence the table is a compound one — i.e., all 

 three of the dental tissues are in wear on it. 



The infundibulum, or mark, being conical 

 in shape, wears with the tooth, becomes smaller, 

 and ultimately vanishes. For example, the 

 permanent central incisors are up at three 

 years ; in the lower jaw the infundibulum 

 wears away about one third in each year ; the 

 animal will probably, therefore, be six years 

 of age when the mark is gone from the central 

 incisors. Sometimes the cavity is absent, its 

 place being occupied by crusta petrosa ; still, 

 in the centre of the tooth there is the inner 

 ring of enamel. Thus, in the transverse sec- 

 tion of a young tooth, we notice an outer and 

 an inner ring of enamel, the interspace being 

 filled by a mass of dentine, and the inner ring 

 lined by crusta petrosa (see Fig. 96). 



In the centre, the table in front of the 

 infundibulum is broader than that behind it, 

 and as the tooth wears it still broadens. In this space 

 sometimes a spot is apparent which differs from the rest of 

 the dentine ; it is the osteo-dentine covering the pulp cavity ; this 

 object has been termed the dental star. The corner teeth may 



I'lG. 9G. 

 Incisor tooth of a 

 Horse — posterior view. 

 a. Outer layer of enamel ; 

 &, Inner layer of enamel 

 round the infundibulum ; 

 0, Dental star ; d, The 

 dentine. 



