450 PHYSIOLOGY. 



posterior leg makes a simultaneous movement with the left anterior and 

 right intermediate. This contemporaneous motion of the several legs 

 does not generally take place so exactly synchronally, but rapidly in 

 succession, so that all the legs of each side are occupied in a constantly 

 progressive advance. The anterior and posterior legs appear to take the 

 greatest share in this advance, and the intermediate ones seem only 

 to support them. Thus, the anterior and posterior legs of one side and 

 the intermediate of the opposite side appear to progress together. But 

 this successive motion is distinctly visible only in insects walking 

 slowly, but, when running, the interval of time is so short between the 

 movements of each individually that the contemporaneous motion of 

 different legs is scarcely perceptible, and we can only discern an alter- 

 nating advance and remaining behind of the two legs of one pair as 

 well as the rapidly successive advancing motion of all the legs of one 

 side. 



In this motion of the entire leg every joint, each in its particular 

 manner, participates, so that the hips revolve upon their axis, the femur 

 approaches to it, and the angle between the femur and tibia becomes 

 more acute when the leg bends, whereas all lie more in a line when the 

 leg is extended. The anterior legs, however, bend in an opposite direc- 

 tion to the rest, for they are extended when they advance ; the others, 

 on the contrary, upon the same movement are bent, whereas, if the 

 posterior ones extend themselves, the anterior pair must necessarily 

 bend. Hence arises the differences in the insertion of the muscles in 

 the hips. The chief object in the anterior pair, namely, is the advanc- 

 ing motion and clinging, but in the posterior pair it is a pushing 

 forward, which is attained by means of extension. Thence the anterior 

 and posterior legs have more extensors than flexors (at least in Melo- 

 lontha, see 17^) '> whereas the intermediate participate chiefly in the 

 advancing motion of the anterior legs of the opposite side, and have 

 consequently more flexors than extensors: an advance forwards is espe- 

 cially prescribed to them, we therefore find their flexile apparatus more 

 developed than the extensional. 



264. 



Leaping, also, is in general effected by means of the legs ; but, as 

 exceptions, we find peculiar organs and apparatus adapted for the 

 purpose. 



When the legs leap it is again the posterior ones which produce the 



