RODENTIA; 



OR GNAWING ANIMALS. 



' HE RODENTS, or gnawing animals, derive their name from the peculiar structure 

 of their teeth, which are specially fitted for gnawing their way through hard sub- 

 stances. The jaws of the Rodents are heavily made, and very large in propor- 

 tion to the head, their size being not only needful for the support of the 

 gnawing teeth, but for their continual development. There are no canines, but 

 a wide gap exists between the incisors and the molars, which are nearly flat 

 on their surfaces, and are well suited for grinding the soft substances on which 

 these animals feed. 



The structure of the chisel-edged incisor teeth is very wonderful, and may be easily under- 

 stood by inspecting the teeth of a rat, mouse, hare, or rabbit. 



As their teeth are continually worn out by the severe friction which they undergo, there 

 must needs be some provision for renewing their substance, or the creature would soon die of 

 starvation. In order to obviate this calamity, the base of the incisor teeth pass deeply into the 

 jaw-bone, where they are continually nourished by a kind of pulpy substance from which 

 the tooth is formed, and which adds fresh material in proportion to the daily waste. Some- 

 times it happens that one of the incisor teeth is broken or injured by some accident, so that it 

 offers no resistance to its corresponding tooth in the opposite jaw. The result of such an 

 accident is very sad to the sufferer, and is not unfrequently fatal in its termination. For the 

 unopposed tooth, being continually increased by fresh substance from behind, is gradually 

 pushed forward until it attains an enormous length, having sometimes been known to form a 

 complete circle. Examples of these malformed teeth are of tolerably frequent occurrence, and 

 specimens may be seen in almost every museum of comparative anatomy. 



Something more is needed for the well-being of the animal than the mere growth of its 

 teeth ; for unless their chisel-like edges were continually kept sharp, they would be of little 

 use for cutting their way through the hard substances which the Rodents are in the habit of 

 gnawing. This result is attained as follows : 



The enamel which covers the front face of the incisor teeth is much harder than that 

 which is laid upon the remaining surfaces, while the dentine which makes up the solid 

 mass of each tooth is also harder in front than behind. It is evident that when these 

 teeth are employed in their usual task, the softer enamel and dentine are worn away very much 

 more rapidly than the remainder of the teeth, so that the peculiar chisel edge of the teeth 

 is continually preserved. Following perhaps unconsciously the structure of these teeth, 

 our cutlera have long been accustomed to make their axes on the same principle, a thin 

 plate of steel being inclosed within two thick plates of iron, so that when the axe is used 

 upon timber, the iron is continually worn away, leaving the plate of steel to project, and form 

 a sharp cutting edge. 



In many species of Rodents, the front faces of the incisor teeth are tinted with a light 

 orange red, or a reddish-brown, by means of a very thin layer of colored enamel. In order to 

 enable these teeth to perform their office rightly,, the lower jaw is jointed so as to slide back- 

 wards and forwards. 



The Rodent animals are widely spread over the entire globe, and are very numerous, 

 comprising nearly one-third of the mammalia. 



The magnificent work on "The North American Rodents," by Dr. Coues and J. A. Allen, 



