x] of the Nervous System. 275 



" often dreadful meteors, namely winds, whirlwinds and thunder. 

 " In the same way the animal spirits, so long as they are pure and 

 " are carried in the open spaces of the brain and its appendages, 

 " behave tranquilly enough, but when shut up within muscles r 

 " and these permeated with sulphurous particles from the blood, 

 "and sometimes with heterogeneous matter in other places, 

 " become exceedingly impetuous, that is elastic, or spasmodic." 



Though he hugged this idea of the animal spirits, the basis 

 of the corporeal soul, being of the nature of light, Willis was 

 no physicist. He caught up the phrases of his friends, Boyle 

 and others, without understanding them, and when he comes 

 to explain nervous phenomena, he mixes up the properties of 

 light with other physical phenomena, and indeed with chemical 

 phenomena. He speaks of the lighter, more spirituous parts 

 of the blood, as ascending by the carotid and vertebral arteries, 

 and as being distilled in the brain, and so prepared, as in a 

 chemical operation, into animal spirits. These animal spirits 

 are prepared in the cerebrum and cerebellum alone, in the 

 cortex of each, and thence diffused over the whole nervous 

 system. There are different kinds of animal spirits; those 

 prepared in the cerebrum are destined for voluntary movement 

 and sensation, those in the cerebellum for involuntary move- 

 ments, for the beat of the heart, respiration and the like. 

 These latter are simple, and have not the diversity of voluntary 

 movements, hence the folds of the cerebellum, unlike those of 

 the cerebrum, are all alike. 



When, however, he discusses what we may call the general 

 phenomena of nervous action he has recourse to the physical 

 phenomena of fluids. The animal spirits pervade the whole 

 nervous system, but in a special way. The nerves are not 

 tubes along which the spirits flow, but solid fibres, and the 

 animal spirits pass along them as spirits of wine pass along the 

 stretched dry strings of a fiddle. This is how he explains 

 nervous action: 



" The internal and immediate efficient cause both of sense 

 " and movement is furnished by the basis of the sensitive soul, 

 " that is to say, by the animal spirits instilled into the brain 

 " from the blood which is alight, and thence diffused into the 



18—2 



