118 



THE STRUCTURE OE THE HORSE 



pointed at the apex, and presenting nothing resembling 

 the pit or infundibulum. They have a tendency to curve, 

 the concavity being turned backwards. Their outer 

 surface is smoothly convex ; their inner surface has a 

 prominent, rounded, longitudinal ridge, and a groove on 

 each side. The borders separating their outer and inner 

 surfaces are, when unworn, sharp and cutting, and meet 

 at the apex. 



Diastema. — Isolated as the canines are from the inci- 

 sors in front, they are separated by a still wider interval 

 (or ' diastema ' ) from the molar teeth behind. This 

 toothless interval, called the c bar 9 in the lower jaw, is 

 of essential importance in the domesticated horse to his 

 master, as without it there would be no room for the 

 insertion of the special instrument of subjugation to his 

 commands — the bit . In the most primitive condition of 

 dentition there appears to have been no such interval, 

 all the teeth being in contact ; and this condition is re- 

 tained, or perhaps regained, by man, almost alone among 

 existing mammals. Already, in Phenacodus, there was 

 an indication of this diastema, and throughout the whole 

 series of Perissodactyles which lead up to the Eqaidce 

 there has been a gradual increase of its length. 



Molar Teeth. — The cheek-teeth, or molars, excluding 

 the rudimentary and inconstant anterior premolar, 

 spoken of before, are six in number, above and below, 

 on each side (see fig. 16, pm 2 to m 3 ). They are all in 



