o/ the Animals of the Farm. 7 



Attention is, in the first place, directed to the upper surface 

 of the crown of the incisor, which is described as the table of 

 the tooth. In tlie centre of the table is a cavity (a) familiarly 

 known as the " mark," on account of the dark colour of its 

 interior. 



In form, the table of the incisor tooth may be described as 

 an elliptical figure, with its long axis running transversely. 

 At the end of the fang the figure is also elliptical, but the long 

 axis is exactly at right angles to that of the table ; and by 



Fig. 2. — Permanent Incisor (Horse). 

 a 



grinding, or by the slower process of wearing an incisor tooth 

 from the upper surface towards the fang, a series of figures will 

 be formed, passing from the elliptical to the oval, the square, 

 and the triangle, first with equal sides, and then with two sides 

 longer than the base. 



For the purpose of making the nature of these changes evi- 

 dent, the outlines of the figures, which result from the wear of 

 tooth structures, are placed on the right of the tooth (Fig. 2). 

 There are, however, other points which require to be noticed. 

 The cavity in the centre of the table is formed by an inflexion 



