92 VERTEBRATED ANIMALS. 



direction, because the principal force exerted by the 

 muscles has already been exerted in the motion from r 

 to ?7i, in bringing the tail in a straight line with the body ; 

 and the force which carries it on to Z, is much weaker, 

 and therefore occasions but a feeble re-action of the 

 water. When the tail has come to /, a similar action 

 of the muscles, on the other side will give an impulse 

 in the direction of k, I, and a motion of the whole fish 

 in the same direction, that is in the line c, a. The flex- 

 ures, and consequent re-action of the tail being repeat- 

 ed in quick succession, the fish moves forwards in the 

 diagonal of c, d, intermediate between the direction of 

 the two forces. 



By bending the whole body almost to a circle, and 

 then suddenly straightning it, fishes are able to leap out 

 of the water, or to ascend a perpendicular cataract of 

 considerable height. 



Did the plan of this little work allow an extension of 

 the subject of this section, it could be shown that the 

 spines of the frog, tortoise, birds, and indeed, all other 

 vertebrated animals present a striking similarity in their 

 structures, and that their forms, lengths, and peculiari- 

 ties are deviations from the general plan we have de- 

 scribed, only so far as is necessary to adapt them to the 

 general organization of the animals to which they be- 

 long. However ignorant any one may be of anatomy, 

 he will generally distinguish the back bone of any animal 

 without mistake, so great is the similarity in all. 



STRUCTURE OF BIRDS. 



In no class of animals is the structure of the several 

 parts, so obviously adapted to the uses for which we see 

 them employed as in the Birds. In these animals, the 

 frame of the skeleton, the figure, position, and construc- 

 tion of the wings, the size of the muscles ; the lightness 

 of the whole system when compared with the size, to- 



In what directions does the spine of a fish allow of motion ? Would 

 any other motion assist the fish in its progress T Explain fig. 69, and 

 show in what manner the fish gains progressive motion through the 

 water 1 What is said of the peculiar adaptation, of the construction of 

 birds to the element in which they move 1 



