378 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 Muacrauchenia. 
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 
a b ie 
Fic. 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 filling the 
cavities and 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 
