RUNNING* 223 



a thick cartilage, possessing very great elasticity. In 

 fishes, whose motions require a great elasticity, this car- 

 tilage is exceedingly thick. In the great basking shark, 

 to increase this elasticity of the spine, there is in the 

 centre of the cartilage, a cavity containing a small quan- 

 tity of water. This is so strongly compressed, that in 

 one instance when the cartilage was punctured, it was 

 projected in a large stream four feet high. 



Emily. It is by this sudden and powerful extension 

 of the body, I suppose, that salmon, trout, and other 

 fishes are able to ascend waterfalls and other obstacles 

 to their course, though many feet high. 



Dr. B. Yes, and this fact shows very forcibly the 

 great elasticity of their spine. 



Leaping is the method of progression used by ani- 

 mals whose anterior and posterior extremities are of dis- 

 proportionate length. The hare, rabbit, jerboa, and 

 squirrel, whose hind legs are so much longer than the 

 others, as to unfit them from running, advance by a suc- 

 cession of leaps. 



Emily. What wonderful leaps a frog will take. 

 Grasshoppers, whose legs are very long in proportion to 

 their body, will makte astonishing leaps for such little 

 creatures. I have seen it calculated that the height to 

 which a grasshopper leaps, is to the length of its body, 

 as 200 to 1 , and a flea, I believe, leaps still farther and 

 with greater velocity. 



Dr. B. The leaping powers of the flea are wonder- 

 ful, and have been thought worthy to be made the subject 

 of a learned work by Roberval, entitled de saltu pulicis, 

 or leaping of the flea. 



Running seems to be a combination of both walking 

 and leaping. It differs from the latter, for the reason 

 that the feet do not move together, and from the former, 

 because the foot behind is raised from the ground before 

 the other has reached it ; so that during every step, the 

 body is suspended for a moment without any support. 



