EQUIDA 395 
thickened ring encircling the upper part, called coronary cushion 
(13), and the sole (15), are covered with numerous thickly set 
papille or villi, and take the greatest share in the formation of the 
hoof ; the intermediate part constituting the front and side of the 
foot (14), corresponding with the wall of the hoof, is covered with 
parallel, fine longitudinal lamine, fitting into corresponding depres- 
sions in the inner side of the horny hoof. 
The horny hoof is divided into a wall or crust consisting of the 
front and sides, the flattened or concave sole, and the “frog,” a 
triangular median prominence, notched posteriorly, with the apex 
turned forwards, situated in the hinder part of the sole. It is 
formed of pavement epithelial cells, mainly grouped in a concentric 
manner around the vascular papille of the keratogenous membrane, 
so that a section near the base of the hoof, cut transversely to the 
long axis of these papille, shows a number of small circular or oval 
orifices, with cells arranged concentrically round them. The nearer 
the surface of the hoof, or farther removed from the seat of growth, 
the more indistinct the structure becomes. 
Small round or oval plates of horny epidermis called “ chest- 
nuts,” growing like the hoof from enlarged papille of the skin, are 
found on the inner face of the fore limb, above the carpal joint, in 
all species of Hquide, and in the Horse (E. caballus) alone similar 
formations occur near the upper extremity of the inner face of the 
metatarsus. Their use is unknown. 
Behind the joint between the metapodium and the first phalanx 
is a prominence formed by the fatty cushion of the fetlock (18 in 
Fig. 164). On the middle of this is a small bare patch covered 
with thickened epidermis, the ergot or spur, generally concealed 
beneath the long hair which grows around it. This is the function- 
less vestige of the large callous pad found in this situation in the 
Tapir, and in fact in all mammals in which this part reaches the 
ground in walking. 
Dentition.—The dentition of the Horse, when all the teeth are 
in place, is, as stated before, expressed by the formula i 3, ¢ 4, p 4, 
m % = 42. The incisors of each jaw are placed in close contact, 
forming a semicircle. The crowns are broad, somewhat awl- 
shaped, and of nearly equal size. They have all the great peculi- 
arity, not found in the teeth of any other living mammal, of an 
involution of the external surface of the tooth (see Fig. 165) 
forming a deep fossa or pit, the bottom of which becomes partially 
filled up with cement. As the tooth wears, the surface, besides 
the external enamel layer as in an ordinary simple tooth, shows 
in addition a second inner ring of the same hard substance sur- 
rounding the pit, thus of course adding greatly to the efficiency 
of the tooth as an organ for biting tough, fibrous substances. This 
pit, generally filled in the living animal with particles of food, is 
