THE GNAWERS — Order, RODENTIA 
WE come now to one of the most numerous, widespread, 
and familiar groups of mammals, —the rodents, or animals 
that gnaw. They are_ small furry or spiny, vegetable- 
Ringlhnim pe cc by their dentition. The 
eristic and most important teeth are the incisors, — 
the gnawers. These are two only in each jaw (except in the 
rabbits and pikas), and are very long and strong, their roots 
extending back through the jaws to the hinder part of the 
skull, where they have “persistent pulps,” that is, 
the formative organ remains active, so that the 
tooth continues to grow throughout the animal’s life. 
Teeth 
This provision maintains in the teeth the length and strength needed 
for the continual gnawing upon which the animal depends for its liveli- 
hood. It is equally requisite, however, that they should always be sharp, 
and this is provided for by having the general substance of the tooth a 
comparatively soft dentine, while the front is coated with a layer of much 
harder, glasslike material called enamel. The end of the tooth is chisel- 
shaped, and as the dentine wears away faster than the enameled front, a 
sharp cutting edge is always maintained; this, too, wears away somewhat, 
but is steadily pushed forward by the active growth at the root to make 
up for the loss, so that the ro- 
dent always has his tools in 
good order. In the best gnaw- 
ers these incisors are thicker 
than wide, and are often 
brown or yellow, and perhaps 
grooved, upon the outer sur- 
Lower JAW OF A BEAVER, face. Another provision of 
Showing the vast length of the ever growing PORENESY 38) that inside the 
incisor (4), and position of the molars (1), mouth, behind the incisor, 
characteristic of the dentition of rodents. there is a hairy ingrowth from 
404. 
