373 



UNGULATA 



importance in the domesticated Horse to his master, as without it 

 there would be no room for inserting the special instrument of 

 subjugation to his commands, the bit) already existed in the 

 earliest known forms, but has gradually increased in length. The 

 incisors have undergone in comparatively recent times that curious 

 change producing the structure more fully described hereafter, 

 which distinguishes the Horse's incisors from those of all other 

 known animals, with the exception of the extinct Macrauchenia. 

 Lastly, the molars have undergone a remarkable series of modi- 

 fications, much resembling in principle those that have taken place 

 in several other groups of herbivorous animals. Distinctions in 

 form which existed between the premolars, at least in the anterior 

 part of the series, and the true molars have gradually dis- 

 appeared, the teeth becoming all very uniform in the shape and 

 structure of their grinding surface. The crowns of all these teeth 



Fig. 157. — a, Grinding surface of unworn molar tooth of Anchitherium ; b, corresponding 

 surface of unworn molar of young Horse ; c, the same tooth after it has- been some time in use. 

 The uncoloured portions are the dentine or ivory, the shaded parts the cement rilling the 

 cavities ami surrounding the exterior. The black line separating these two structures is the 

 enamel or hardest constituent of the tooth. 



in the early forms were very short (see Fig. 158, a); there was a 

 distinct constriction, or neck, between the crown and roots ; and 

 when the tooth was developing, as soon as the neck once rose 

 fairly above the alveolar margin, the tooth remained permanently 

 in this position. The term " brachydont " expresses this condition 

 of teeth, the mode of growth of which may be illustrated by those 

 of man. The free surface had two nearly transverse curved ridges, 

 with valleys between (Fig. 157, a); but the valleys were shallow 

 and had no deposit of cement filling them, the whole exposed 

 surface of the unworn tooth being formed of enamel. When the 

 ridges became worn down the dentine of the interior was exposed, 

 forming islands surrounded by enamel. With the progress of time 

 the crowns of the teeth gradually became longer, the valleys deeper, 

 and the ridges not only more elevated but more curved and com- 

 plex in arrangement. To give support to these high ridges and 

 save them from breaking in use, the valleys or cavities between 

 them became filled up to the top with cement, and as the crown 

 wore down an admirable grinding surface consisting of patches and 



