On Muscular Motion and Animal Spirits 239 



cause the contraction of the fibrils is effected. There 

 can be no doubt that the influx of animal spirits is 

 necessary for the performance of the motive function, 

 inasmuch as if a nerve is cut or obstructed, the 

 muscle to which it is distributed refuses to contract. 

 But it is not to be supposed that the contraction of 

 the muscles depends on the .animal spirits alone, since 

 for carrying it out there is absolute need of other 

 particles besides, brought from the mass of the blood. 

 For, seeing that the arterial blood is supplied to the 

 muscles in a continuous flow, and, especially in more 

 violent movements, in a fuller flow than to other 

 parts or than is required for their nutrition, it may be 

 concluded that the arterial blood in its transit de- 

 posits something necessary for the contraction of the 

 muscles. For, indeed, such is the structure of muscles 

 that in their contraction they draw themselves 

 together, and thus greatly promote the motion of the 

 blood ; and that is why the motion of the blood is so 

 much accelerated in rapid running. And we can 

 easily put this to the test, if, when the median vein 

 has been opened, the muscles attached to the forearm 

 are contracted (and this can be quite well accomplished 

 if the fingers are pressed together in flexion), for when 

 this is done, the blood will be seen to rush out forcibly, 

 being pressed out of the said muscles by their contrac- 

 tion. But this so much accelerated motion of the 

 blood in contracted muscles does not seem to be 

 merely incidental, but to be arranged by the highest 

 wisdom of nature, by which, namely, the motive 

 particles of the blood may be brought in passing to 

 the motor parts ; and when they have been deposited 

 from the blood, what remains of it is expelled with a 

 certain push by the constriction of the shortened 

 muscle, so that when the loss is repaired, it may again 



