THE MOUTH. 23 



can see the whole of it, you see that it is bent 

 almost in the form of a crescent, (Fig. 4, E). In 

 describing the tooth, we must suppose it divided 

 into two parts ; the visible part and the invisible 

 part. The visible part is all that standing ahove 

 the gums and is called the crown: the invisible 

 part is that imbedded in the jaw and covered by 

 the gums, and is called the fang. By studying the 

 anatomy of a nipper tooth in a rough way, we can 

 judge of a horse's age by the so called ' marks' of 

 his teeth. In Fig. 4, E, is a section down the 

 middle of a nipper tooth from front to back. Let 

 the line a a represent the line of the gums, then 

 all above this is the ' crown' and all below it the 

 ' fang.' The great bulk of the tooth is seen to be 

 made up of the part indicated by the number 3, and 

 is called the dentine. This substance has a coating 

 of a substance termed ' enamel' {E 2), for the part 

 of the tooth ahove the gum or the 'crown,' but 

 wdiere the tooth gets fixed into its bony socket in 

 the jaw, the 'dentine' is not covered by 'enamel' 

 as in the ' crown,' but by a very thin layer of bone, 

 E 6, called ' crusta petrosa.' This is a long hard 

 name, but we have done with it. Now pay attention 

 exclusively to the crown of the tooth (the part above 

 the gum line a a), and you see that the enamel 

 E 2, after reaching the cutting surface of the tooth, 

 dips into the tooth and forms a little sack-like 

 cavity filled up with black material {E 5). Now 

 suppose you cut off with a saw a piece of the cutting 



