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4 The long, sharp claws Post Date: Thu, 14 Aug 2008 22:28:55 +0000 It, more- over, acts as a parachute, breaking the force of its fall, in case the animal should drop down from a tree or be obliged, in fleeing from a pursuer, to take a downward leap from the summit of a tree to the earth. In such an emergency, the animal extends its limbs horizontally from the body in order to present to the air as large a surface of resistance as possible. 4 The long, sharp claws of its toes, by which it can firmly hook itself to the bark of trees, further enables it to climb with a skill unequalled by any other of our native animals. Autor of the post: Undefined This consists of acorns, beech-nuts Post Date: Thu, 14 Aug 2008 22:15:35 +0000 Even smooth-barked trees, such as the beech and birch, it ascends by leaps, with remarkable speed and surety. The trees of the forest provide the animal with its food. This consists of acorns, beech-nuts, nuts, the seeds of the pine and fir, young juicy shoots, buds, berries, fungi, and the bark of branches, according to what the forest or the season of the year can supply. Autor of the post: Undefined By allowing the animal Post Date: Thu, 14 Aug 2008 22:03:25 +0000 For getting at the hard kernels of nuts or gnawing, ie, cutting off in small bits the hard bark of trees, it is provided with 1 Incisors, or gnawing teeth, as they are called, from their mode of action. A pair of these is present in each jaw. By allowing the animal to use its teeth on a slab of wax, the impressions will be found exactly to resemble those made by a chisel, or, rather, a fluting plane. Autor of the post: Undefined For the same reason, also Post Date: Thu, 14 Aug 2008 21:52:40 +0000 We are, in fact, really dealing with small chisels or a planing tool. (a) If it is desired to pick small pieces from the flat surface of a board with a chisel, it is usual to place the tool at as acute an angle as possible (why?), and deal the handle a heavy blow, when a splinter will be detached. For the same reason, also, the incisors of the squirrel should form as acute an angle as possible with the object to be gnawed. Autor of the post: Undefined (c) By constant use, even Post Date: Thu, 14 Aug 2008 21:41:05 +0000 This is effected by their being very strongly curved. (b) The front-teeth, having to perform severe work, must be set deeply and firmly in the jaws ; and, in fact, the parts inserted in the jaws are very long, far longer than those which project ; in the case of the lower in- cisors they extend back behind the molar teeth. (c) By constant use, even the strongest steel instrument is worn away. Autor of the post: Undefined (Why can they not Post Date: Thu, 14 Aug 2008 21:30:18 +0000 In the same way, the incisors of the squirrel wear away in time, and they are constantly renewed at tlieir inner ends in proportion as they are worn away. The portions of the teeth embedded in the jaws are constantly being pushed outwards from their curved sockets ; consequently their curvature must be uniform throughout, forming, in fact, a portion of the circumference of a circle. (Why can they not be straight ?) (d) If the incisors, like those of man, for instance, were covered all round by enamel, they would soon wear away in an equal degree against hard objects, ie, become blunt. Autor of the post: Undefined 2 If the incisors were Post Date: Thu, 14 Aug 2008 21:14:52 +0000 Here again the resemblance to a chisel or plane is complete, for just as, in these instruments, only the cutting- blade consists of steel, so in the rodent teeth only the outer side is coated with a thick plate of enamel ; all the rest of the tooth consists of softer osseous material. By the gnawing of hard objects this osseous material is worn off more easily than the enamel layer of the cutting surface ; the latter consequently remains constantly sharp, and the tooth itself retains its chisel-like form. 2 If the incisors were close to the other teeth, as in man, they would not be fitted for separating a splinter or for penetrating into hard substances, as in opening a hazel-nut or tearing off the scale of a pine- cone. Autor of the post: Undefined There are consequently no canine Post Date: Thu, 14 Aug 2008 21:04:15 +0000 If they were more numerous, the cutting edge would be too broad. The incisors must therefore project freely, and therefore between them and the molars there is a wide gap, or diastema. There are consequently no canine teeth. Autor of the post: Undefined Now, in order Post Date: Thu, 14 Aug 2008 20:45:20 +0000 Nor, indeed, does a herbivorous animal need them. 3 There are five grinding teeth (molars and premolars) on each side of the upper jaw, but the first is rudimentary, and is often last in adult life ; in the lower jaw there are four on each side. Now, in order to be digested, 'all vegetable substances must first be carefully ground up. Autor of the post: Undefined The mandibles in chewing, accordingly Post Date: Thu, 14 Aug 2008 20:34:55 +0000 How is this effected? If the lower jaw were as firmly articulated as in the carnivora (see cat), this would not be possible. In rodents, there- fore, the articulating head or condyle of the lower jaw is elongated longitudinally, and glides along an equally elongated articular groove. The mandibles in chewing, accordingly, move from behind forwards (sliding movement), and the molars thus act as files, which grate up the food. 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