J88 



MAMMALIA. 



is two large and strong incisive teeth in the front of 

 each jaw, those in the lower jaw sloping forward, and 

 the lip often cleft and the nose shortened in order to 

 be out of the way of those in the upper jaw. The 

 other teeth are generally three grinders in each 

 side of both jaws ; but some of the genera have 

 three additional ones which have sometimes been 

 called canines, though they have nothing of the true 

 canine character about them. None of the teeth 

 except the front ones are capable of dividing the 

 food, though they are very efficient in bruising it. 

 The incisive teeth must therefore be considered as 

 the instruments with which the food is taken, though 

 in some species the fore-paws come in to assist. 

 The effective part of the mouth therefore resembles 

 a pair of pincers ; and the muzzle is in most of the 

 species rather short, and the head enlarged on the 

 sides for better insertion of the muscles, so that the 

 jaws may gnaw with great force, and also for a con- 

 siderable length of time. In the individual structure 

 of the teeth there is something curious, as showing 

 how well an efficient cutting instrument among 

 animals agrees with an instrument of the same 

 kind as constructed by man. Man, the handed ani- 

 mals, and the carnassiers of all kinds, have the teeth, 

 in the exposed part, covered wholly with enamel ; 

 and animals which grind vegetable substances with 

 their cheek teeth, have the crowns made up of alter- 

 nate portions of enamel and bone, the last of which 

 wears down a little, and leaves ridges of the other 

 something resembling those of a millstone. The 

 gnawing teeth of the rodentia are different from both 

 of these. They have the enamel placed on their 

 external faces, with a support of bone behind it, 

 which preserves a cutting edge of the enamel, and 

 furnishes a support of a substance which, though not 

 so hard, is tougher; and thus the greatest strength 

 and efficacy jointly are given to those long teeth 

 teeth which have perhaps more severe labour to per- 

 form in proportion to their number than those of any 

 other animal. This is exactly the plan to which a 

 workman has recourse when he wishes to make a 

 tool which shall retain the keenest and most durable 

 edge, and which yet shall have sufficient toughness 

 not to snap by a pretty severe cross strain. A thin 

 piece of that description of steel which becomes 

 hardest by tempering, is applied and fastened to the 

 side of a thicker piece of iron. Then that which is 

 to be the cutting edge is chamfered off on the face 

 which has the iron to form basil ; and thus while 

 there is steel enough for all the purposes of cutting, 

 the iron supports it up to the very edge, so that the 

 tool can cut finely, and yet bear pretty rough usage. 

 There is another advantage to a tool of this kind : it 

 is more easily sharpened than if the whole of it were 

 formed of even softer steel, because the iron is ground 

 down much more easily than steel would be. This 

 application of the two substances in the teeth, has the 

 same advantage in the sharpening of them, or rather 

 it keeps them sharp by the very fact of using them ; 

 for though the enamel does wear, the soft bone be- 

 hind it has a tendency to wear faster, and by this 

 means the edge of the tooth never gets blunt, let the 

 animal gnaw as it will. There is also a provision 

 against the wearing down of the teeth in their con- 

 tinuing to gnaw during the whole life of the animal, 

 which is not the case with teeth that are entirely 

 cased with enamel. The fact has not been absolutely 

 decided, but we have reason to believe that the growth 



of the tooth is in proportion to its wearing, and con- 

 sequently to the use that is made of it. This, how- 

 ever, is not absolute ; because if one of the teeth in 

 either jaw is lost, the corresponding one in the other 

 jaw grows so long that the animal can use the 

 mouth with difficulty. There is another circumstance 

 which assists in this operation, and that is the form 

 of the condyles by which the lower jaw is articulated. 

 These are elongated from front to rear ; and thus, 

 though they admit the point of the under jaw to be 

 advanced or drawn back as the operation of gnawing 

 may require, they do not admit of any lateral motion. 

 This gives great firmness to the bite of these animals ; 

 and thus, though their teeth are not calculated for 

 inflicting punctured wounds as the canine teeth of 

 animals are, they can bite out a piece equal to their 

 extent much more quickly than -Any of the teeth which 

 are formed for eating flesh. The cheek teeth have 

 flat crowns with transverse ridges of enamel, and the 

 interstices filled up with bone. These ridges in the 

 opposite jaws nearly fit into each other, so that the 

 grinders can act something after the fashion of the 

 hand instrument which was used for breaking the 

 " boon " of lint before the more expeditious and success- 

 ful expedient of rollers was resorted to. The crowns of 

 the cheek teeth are not, however, exactly of the same 

 form in all the rodentia. If these crowns are perfectly 

 flat, with only the exception of the transverse ridges 

 of enamel, the owners live exclusively upon vegetable 

 substances. If the ridges are divided into smooth 

 tubercles, the owners are more miscellaneous in their 

 feeding, and eat indiscriminately a variety of sub- 

 stances ; and if the tubercles are drawn up into sharp 

 points, their owners possess a little of the disposition 

 and habits of the carnassiers. 



The extremities of the rodentia are seldom of equal 

 length, the hind legs being longer than the fore ones 

 in most of the species. In proportion as these are 

 longer, the muscular substance and power of the ani- 

 mal is more concentrated upon them. Their motion 

 is, generally speaking, leaping and not walking, even 

 when it is slow: and those which have the hind legs 

 very long can leap to great distances compared with 

 the size of the animals. 



Though exclusive vegetable feeders in many of the 

 genera, the rodentia have only a single stomach ; but 

 this deficiency is often made up by a co3cum of very 

 large size. The dormice, however, have not this 

 appendage. 



The brain, in all the order, is but little marked with 

 convolutions on its surface ; and as those convolutions 

 are popularly supposed to be some way or other con- 

 nected with sagacity and resources in proportion to 

 their development, it is customary to say that persons 

 who have little sagacity or resource, and are giddy 

 and wavering, are hare-brained. The eyes are placed 

 laterally, and the form of the zygomatic or yoke 

 arches shows that the jaws are but weak. Strength 

 in the jaws is not the requisite, it is rather lightness, 

 because the action is continued gnawing or nibbling, 

 and not powerful biting. This may be seen in the 

 different ways in which a monkey and a squirrel go 

 about to get at the kernel of a nut. The monkey 

 takes it between his strong jaws and cracks it at 

 once by one lusty gripe ; while the squirrel nibbles, 

 away till it makes a hole in the nut. As the fore 

 arms have but little motion in the elbow joints, and 

 their two bones are often united, the paw cannot 

 turn, but has merely a hinge motion in this particular 



