On Muscular Motion ajtd Animal Spirits 299 



rotated about a^ as a centre ; I say that while it is 

 moving in a circle, all the parts of it, say c, 3, strive 

 as far as in them is to recede from the centres of the 

 circles which they describe, and tend to move in the 

 straight lines, c, e^ and ^, e : for a similar argument 

 holds here as in the case of a stone rotated in a sling, 

 which always strives to get out of the sling and to 

 begin a movement in a straight line, as has long ago 

 been noted by the acute Descartes. But now if we 

 suppose that while the stick is in rotation it be set 

 free from the centre, a^ when it has come to a 

 horizontal position, it will no longer move in a circle, 

 but ascend straight up. These things being premised, 

 let (in Plate III., Fig. 7), a^ e^ be the tibia, c, ;z, the 

 thigh-bone, e^ b^ e^ the muscles which extend the tibia ; 

 when the animal is in the attitude for jumping, the 

 thigh-bone, <:, ;z, by the contraction of these muscles 

 will be rotated about the protuberance of the tibia at 

 ^, just as the aforesaid stick was rotated about the 

 centre. Whence it comes about that the thigh-bone 

 thus circumgyrated will strive to move up ; and 

 indeed if it has been driven round with sufficiently 

 brisk motion by these muscles, that effort will attain 

 its end, and the thigh-bone will carry the tibia aloft 

 with it. And the case is quite similar with the 

 muscles extending the foot, the tibia, the thigh, and 

 the back : when they contract strongly, the parts 

 into which they are inserted are driv^en in a circle 

 and carried upwards, as is seen in Plate III., Fig. 8, 

 which shows the muscles extending the foot, the 

 tibia, the thigh, and the back, with the bones into 

 which they are inserted. The flexor muscles of the 

 arm, the forearm, and the hand have been drawn in 

 the figure because these muscles contribute not a 

 little to the lifting of the body, in so far of course as 



