262 Mayow 



we may believe that the reason why the said oils are 

 useful in these diseases is because they are specially 

 suitable for strengthening the motor parts and the 

 nervous fibres. For it has been found by experience 

 that those things which consist of volatile salt and 

 volatile sulphur are specially suitable for restoring the 

 fibres of the muscles to a proper tone. Whence it 

 comes that medicaments of that sort bring help to 

 the weakened and all but broken membranes of the 

 brain, and consequently afford very great aid in the 

 above-mentioned diseases. 



But, to go a little further in our conjectures as to 

 the use of the said membranes, it seems probable that 

 sleep is caused by the membranes surrounding the 

 brain, but not those surrounding the cerebellum, either 

 ceasing from their motion or at all events performing 

 it but remissly, so that the nitro-aerial particles are 

 no longer carried into the brain, and thus the animal 

 functions are necessarily interrupted. For that a 

 need of sleep presses upon animals wearied with 

 labours is not, we must suppose, because of a want of 

 nitro-aerial particles, i.e,^ of animal spirits, inasmuch 

 as a never-to-be-exhausted stock of them exists in the 

 air, but rather because the saline-sulphureous particles 

 of the blood have been consumed by watching and 

 work ; but the deficiency of saline-sulphureous particles 

 concerns the motor parts but not the brain, except in 

 so far as the meninges of the brain, which are to be 

 reckoned among the motor parts, refuse to perform 

 their pulsations if there is a want of saline-sulphureous 

 particles in the blood. Wherefore it would seem that 

 we should hold that sleep takes place because the 

 saline-sulphureous particles are so much consumed by 

 work and watching that, on account of their deficiency, 

 the meninges of the brain refuse to perform their 



