494 THE HORSE'S POSITION IN THE ANIMAL WORLD 



is a fact that there are the same number of bones and a simihxr arrange- 

 ment of them, and in short a general uniformity of the plan of con- 

 struction in l)oth cases, varied in details under the influence, it may be 

 presumed, of the conditions of existence. 



With regard to the teeth of the horse some points of considerable 

 interest have to be noticed. It has already been stated that the horse 

 has six front teeth or incisors, named more correctly, from their flat sur- 

 faces, nippers, in each jaw, four canine teeth {tusks) in the male, two on 

 •each side, top and bottom, while in the female the tusks are absent or 

 ^ire in a rudimentary condition. There are also six molars on each side, 

 top and bottom. The last three of these are specially distinguished as 

 the molars, those in front being known as p7-emola7-s. To these must 

 be added the first premolars, eye-teeth or wolves'-teeth, in the upper jaw, 

 which are in a rudimentary state, and have disappeared entirely from 

 the lower jaw. In the ancient ungulate mammals the first premolars 

 were fairly well developed teeth, making a row of seven instead of six 

 molars. After a gradual diminution in size, which may be traced in 

 the fossilized remains of the ungulates of the tertiary formation, these 

 teeth are represented in the horse of the present time only by the small 

 conical teeth in front of each of the first well - developed molars of the 

 upper jaw. These teeth (the eye-teeth) — which, as is well known, were 

 once (and are still by some) looked upon as a cause of blindness, and 

 were always punched out as soon as discovered — are undoubtedly there- 

 fore vestigial remains, and in course of time may cease to appear altogether 

 in the horse's mouth. They are usually got rid of between two and 

 three years of age, when the two first molars are exchanged for the 

 second teeth or permanent molars. 



Between the corner incisors and the first molars is a clear unoccu^jied 

 space, the diastema, popularly called the har. This toothless space did 

 not exist in the most ancient mammals, but in the primitive equine 

 ungulates there were some indications of it, and the feature becomes 

 more and more distinct throuoh the whole series of horse-like animals. 

 What circumstances led to the change, or what olyect is gained by it, 

 is not known, but in the horse the space in the lower jaw is taken ad- 

 vantage of for the purpose of adjusting the bit. The incisors and molars 

 of the horse are remarkable for their complicated structure. Three 

 materials of different degrees of density may be distinguished in their 

 formation. Of these the least dense is known as the crusta jwtrosa 

 or cementum; the next in hardness forms the bulk of the tooth, and 

 is called dentine; the hardest of the three is the enamel Avhicli covers 

 the dentine, following it in all its convolutions. The crusta petrosa 



