300 Mayow 



they, when violently contracted, forcibly raise up the 

 parts annexed to them, as any one can feel for 

 himself when he jumps. But this is to be noted 

 here, that these bones along with the parts attached 

 to them are not carried straight up by the contraction 

 of the said muscles, but that the thighs are carried up 

 and forwards, while on the other hand the back is 

 carried up and backward, as is made clear in the 

 figure ; yet so that in the motion of animals moving 

 straight up, the force which carries some parts 

 forwards and that which carries other parts backwards, 

 being in equilibrium, mutually destroy one another, 

 while the other motion by which all the parts are 

 carried upwards is unimpaired ; but as the animals 

 jump forwards or jump backwards so must the force 

 by which some parts are directed forwards, or that by 

 which others are directed backwards, prevail. 



But that this may be still more clearly made 

 apparent, I say the said extensor muscles raise the 

 animal aloft in the same way as anything is thrown 

 from the hand. 



For as to the force impressed upon things projected^ 

 it is not to be thought of as anything else than 

 motion alone : when a thing is once set in motion it 

 will continue in it until it is retarded by bodies in the 

 way, or by the gravity inherent in itself ; for, since 

 (as has been noted by the acute Descartes), if a thing 

 is at rest we do not believe that it will ever begin to 

 move unless it is impelled to do so by some cause, 

 so there is no reason why we should suppose that if 

 a thing is moving it should intermit its motion of its 

 own accord, and not as hindered by something else. 

 For as inanimate things obviously cannot dispose of 

 themselves, but are altogether subject to the disposi- 

 tion of others, they must continue in the state in 



