i895. THE TEETH OF THE HORSE. 251 



third deciduous incisor and canine not yet having appeared. At 

 birth these teeth are separated by an interval of an inch and a half, 

 and the diastema attains its full width of three to four inches in 

 between one and two years. 



With this late development of the facial parts of the skull is 

 connected the relation of the last-formed cheek tooth to the orbit. 

 In a half-grown foetus, a vertical plane through the anterior margin of 

 the orbit passes between the last two deciduous molars ; at birth it 

 passes between the last deciduous molar and the first molar, at one 

 year between the first and second molars, at two to three years 

 between the second and third molars, and at fifteen years or so 

 behind the last molar. Each of the last four upper cheek teeth thus 

 begins to develop under the orbit, but, as it increases in size, is 

 gradually brought more and more forward by the lengthening of the 

 maxillary bone. 



The first and second deciduous incisors and the three deciduous 

 molars appear to cut the gum together, shortly after birth ; but the 

 third deciduous incisor is backward, and only makes its appearance 

 above the gum at eight or nine months. All the milk teeth are thus 

 in use before the end of the first year. At five years all the milk teeth 

 have been shed, the permanent teeth are in position, and the horse is 

 said to have a " full mouth." The dates mentioned in these, and in the 

 subjoined remarks, are naturally only approximate, since much depends 

 on the breed and physique of the horse, the character of the food, and the 

 fact that the upper teeth usually cut the gum a little before the lower. 



The incisor teeth are known to veterinarians by special names : 



those nearest the middle line are called the " centrals " or " pincers " ; 



those lying on the outer sides of the centrals are the " intermediates " ; 



while the third incisors of comparative anatomy are called the 



" corners." The anterior (labial) edges of these teeth are disposed in 



a curve, which, up to the age of five years, is nearly semicircular, but 



which later in life becomes gradually flattened out, the lower centrals 



and intermediates being in a straight line at twelve years. Each 



incisor tooth is slightly curved, the concavity being directed towards 



the mouth, and it tapers uniformly from crown to root. That surface 



of the tooth which is continually wearing away by constant friction 



with food material and with the opposing tooth, is called the " table." 



The table of a young incisor bears a central depression or cavity, 



called the external dental cavity or " infundibulum " (*., Fig. J.) ; this 



is usually filled with blackened food-stuff and constitutes the "mark" 



of the tooth. The enamel of such a tooth is disposed in two layers : 



one, the " central enamel," lining the infundibulum; and the other, the 



" peripheral enamel " surrounding the dentine. Before the tooth cuts 



the gum the two layers of enamel are continuous at the margin of the 



infundibulum, and it is only when the tooth bites with its opponent in 



the other jaw that this rim of enamel becomes eroded and the dentine 



exposed. The external layer of enamel, more especially the embedded 



t 2 



