SPECIAL ANATOMY. 129 



the pulp cavity, for it contains a small papilla to wliicli an 

 artery and a nerve run and from wliicli a vein proceeds ; 

 this is the pulp. It is immediately surrounded by dentine, 

 which is formed continuously from its outer part, so that 

 the pulp cavity, which is large in the young animal, in the 

 adult becomes almost obliterated ; on the outer surface of 

 the dentine rests a layer of very dense white substance of a 

 petrous nature, enamel. It is deficient on the lower part 

 of the fang. Externally placed to this we see a yellowish- 

 brown matter, crusta petrosa or cementum.* The upper 

 surface of the tooth generally presents one or more depres- 

 sions, the infundibuli, into which dip the three layers of 

 substance of which the tooth is composed, and which are 

 frequently filled with crusta petrosa. When a tooth is 

 wholly covered on its external surface with enamel it is 

 termed a simple tooth, all other teeth are compound. We 

 see simple teeth in the dog ; the canines of the horse are 

 by some considered simyjle. Teeth are named according to 

 their function, grinders and pincers. These are arranged 

 in rows, each of which consists of six teeth, and between 

 the rows of grinders and pincers in the horse are situated 

 solitary teeth, the canines, cuspidati, or tushes, which in the 

 mare are rudimentary; they are weapons of offence or of 

 defence, and are well developed in stallions. The incisors 

 or pincers are situated in the mouth, anteriorly in contact 

 with the inner surface of the lips, arranged along the 

 anterior crescentic margin of the hard palate ; a corre- 

 sponding number being placed in the upper margin of the 

 lower jaw, three incisors on each side of the symphysis. 

 The upper incisors are larger, longer, and put in their 

 appearance later than those of the lower jaw. The cutting 

 surface of the teeth is ovoid, with an acute angle at either 

 extremity ; the anterior edge is higher than the posterior, 

 and the tooth, taken as a whole, presents a curve the con- 

 cavity of which is posteriorly situated. There are two 

 complete sets of incisors, temporary or milk, and perma- 



* Crusta petrosa in structure exactly resembles bone presenting 

 laminse, lacunse, and canaliculi, and, where it is very thick, Haversian 

 canals. Dentine consists of an earthy matrix, through which branch- 

 ing and wavy canaliculi run from the pulp cavity, and near its outer 

 surface terminate in lacunse. Enamel consists of prismatic hexagonal 

 bodies, which have their margins jagged to enable them to fit closely 

 together. One extremity of these looks towards the pulp cavity, the 

 other towards the surface of the tooth. 



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