as indicative of the Age of the Animal. 
281 
twitching movement of the head. These several actions of the lips, 
tongue, and teeth, in taking hold of the food and conveying it into 
the mouth, are variously modified, being, as we have seen, even 
more marked in some of our domesticated animals than in others. 
The incisor teeth, although always placed in the front of 
the mouth, are differently arranged in different animals. In the 
horse and also in the pig these teeth are twelve in number — six 
being placed in the upper and six in the lower jaw, so that in the 
act of biting, their faces or wearing surfaces oppose each other. 
In the ox and sheep, on the contrary, the incisors, which are eight 
in each of these animals, are all placed in the lower jaw. They are 
opposed by a dense yet somewhat elastic cushion attached to the 
upper jaw, which has been named the dental pad (see d. p., fig. 
35). The power of resisting the pressure of the incisors is given 
to the pad from its being chiefly composed of white fibrous tissue : 
mingled, however, with this is some yellow or elastic tissue, which 
gives to it that small amount of yielding to the force employed, 
which it likewise possesses. Upon the surface of these struc- 
tures a capillary network of blood-vessels is laid for the purpose 
of forming that dense cuticular membrane which is observed to 
cover the pad in common with the whole of the inner surface of 
the mouth. This membrane, v/liile it protects the sensible parts 
within, by resisting, like the cuticle of the true skin, the effects of 
attrition from without, gives increased firmness to the dental pad. 
The existence of this elastic cushion in the place of the upper 
teeth is rendered the more necessary in ruminating animals from 
the peculiar form of their incisors. In common language these teeth 
may be said to be chisel-shaped in the ox and sheep, especially 
when first protruding from the gum (see fig. 1). Besides having 
broad crowns which are flattened from before to behind, taper- 
ing to a sharp cutting edge above (fig. 1, a), the incisors of these 
animals have also rounded and comparatively small fangs for 
their size (fig. 1, c). Their fangs are likewise rather loosely 
implanted in their respective sockets, so that a degree of mobility 
exists between each tooth and its socket. These peculiarities 
consequently require a modification of the structures which oppose 
the incisors in their action. If, as in the horse, they were met 
witli an equal number of teeth firmly attached to the jaw by 
their fangs, then it is evident that they would be exposed to dis- 
placement and early removal from their sockets. In grasping the 
herbage and detaching it from its roots, the ox can employ just 
that amount of compression which is needed, and this with per- 
fect safety to the teeth, as both they and the pad yield slightly to 
the force imparted. 
Other peculiarities of these incisors, and the changes which 
they undergo at different periods of an animal's life, will be 
