88 VERTEBRATED ANIMALS. 



direction, because the principal force exerted by the 

 muscles has already been exerted in the motion from r 

 to m, in bringing the tail in a straight line with the body ; 

 and the force winch carries it on to /, is much weaker, 

 and therefore occasions but a feeble reaction 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 /c, /, 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 repeated 

 in quick succession, the fish moves forward in the diagonal 

 of c, d, intermediate between the direction of the two 

 forces. 



250. By bending the whole body almost to a circle and 

 then suddenly straightening it, fishes are able to leap out 

 of the water, or to ascend a perpendicular cataract of con- 

 siderable height. 



251. 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 peculiarities, 

 are deviations from the general plan we have described, 

 only so far as is necessary to adapt them to the general 

 organization of the animals to which they belong. How- 

 ever ignorant any one may be of anatomy, he will gene- 

 rally distinguish the back-bone of any animal without 

 mistake, so great is the similarity in all. 



STRUCTURE OF BIRDS. 



252. In np class of animals is the structure of the seve- 

 ral 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? Explain Fig. 69, and 

 show in what manner the fish gains progressive motion through the 

 water. What is said of the peculiar adaptation of the construction of 

 birds to the element in which they move ? 



