OF THE ANIMAL FRAME. 



119 



of the one part into the other upon which their mutual action depends. The 

 extent and nature of the motion is determined by the nature of the articula- 

 tion, which is varied with the nicest skill to answer the purpose intended. In 

 ostraceous' worms the only articulation is that of the hinge : in the cancer 

 tribes the tendon is articulated with the crust, whence the wonderful strength 

 and activity of the claws ; and it is articulated in a similar manner with the 

 scaly plates of some species of the tortoise. In insects the part received and 

 the part receiving form each a segment of a spheroid ; whence the motion 

 may be either rotatory or lateral, at pleasure. In mammalian animals the 

 lower jaw only has a power of motion ; but in birds, serpents, and fishes, the 

 upper jaw in a greater or less degree possesses a similar power. 



The motion of serpents is produced, according to Sir Everard Home, by their 

 ribs, which for the most part accompany them, not only as organs of respira- 

 tion, but from the hind extremity to the neck, and are possessed of a peculiar 

 power of motion by means of peculiar muscles. " The vertebrae are articu- 

 lated by ball and socket joints (the ball being formed upon the lower, and the 

 socket on the upper one), and have therefore much more extensive motion 

 than in other animals." In the draco volaiis the skeleton of the wings is 

 formed out of ribs which " are superadded for this purpose, and make no part 

 of the organs of respiration ; the ribs in these animals appear to work in suc- 

 cession, like the feet of a caterpillar." 



The TEETH vary in their form and position almost as much as the bones. 

 Where jaw-bones exist the57^ are usually fixed immoveably in their sockets ; but 

 in some animals a few of them are left moveable, and in others the whole. 

 The mus maritimus, or African rat, the largest species of this genus which 

 has hitherto been discovered, and seldom less than a full-sized rabbit, has the 

 singular property of separating at pleasure to a considerable distance the two 

 front teeth of the lower jaw, which are not less than an inch and a quarter 

 long. That elegant and extraordinary creature the kangaroo, which, from 

 the increase that has lately taken place in his Majesty's gardens at Kew, we 

 may soon hope to see naturalized in our own country, is possessed of a simi- 

 lar faculty. And the hollow tusks or poisoning fangs of the rattlesnake, and 

 other deadly serpents, are situated in a peculiar bone on each side of the 

 upper jaw, so articulated with the rest, that the animal can either depress or 

 elevate them at his option. In a quiescent state they are recumbent, with 

 their points directed inwards ; but whenever the animal is irritated he in- 

 stantly raises them ; and at the moment they inflict a wound, the poison, 

 which lies in a reservoir immediately below, is injected through their tubes 

 by the act of pressure itself. 



In the shark and ray genera the whole of the teeth are moveable, and lie 

 imbedded in jaw-cartilages instead of in jaw-bones, and like the fangs of the 

 poisonous serpents are raised or depressed at pleasure. The teeth of the 

 xiphias gladius, or sword-fish, are similarly inserted ; while his long sword- 

 like snout is armed externally, and on each side, with a taper row of sharp, 

 strong, pointed spines or hooks, which are sometimes called his teeth, and 

 which give rise to his popular name. 



The ant-eater and manis swallow theii: aliment whole ; and in many ani- 

 mals the jaws themselves perform the office of teeth, at least with the assist- 

 ance of the tongue. In birds this is generally the case, sometimes in insects, 

 whose jaws are for this purpose serrated or denticulated at the edge, and fre- 

 quently in molluscous worms. The jaws of the triton genus act like the 

 blades of a pair of scissors. The snail and slug have only a single jaw, 

 semilunar in its form, and denticulated : but the mouth of the nereis has several 

 bony pieces. The sea-mouse (aphrodita aculeata) has its teeth, which are 

 four, fixed upon its proboscis, and is of course able to extend and retract them 

 at pleasure ; and the leech has three pointed cartilaginous teeth, which it 

 is able to employ in the same way, and by means of which it draws blood 

 freely. In like manner, though insects cheifly depend upon a serrated jaw, 

 yet many of them are also possessed of very powerful fangs, of which we 

 have a striking instance in the aranea avicidaria, or bird-spider, an inhabitant 



