2 6o Mayow 



by itself for bringing the nitro-aerial spirits to the 

 brain in ample enough quantity : wherefore I consider 

 it likely that the thicker menmges which surrounds 

 the brain undergoes a sort of pulsation, and that by 

 its contraction the blood driven to the brain is 

 compressed ; and that thus the nitro-aerial particles 

 are pressed out of the mass of the blood, and driven 

 into the brain, in a way not very unlike that in which 

 the other motive particles are forced into the motor 

 parts by the constriction of the muscles. Such a 

 pulse of the dura mater is confirmed by autopsy 

 itself: for, in fracture of the skull, part of the brain 

 comes into view, it is seen to rise in a tumour, and 

 immediately in turn to subside, which seems to be a 

 motion of the brain after the manner of the heart's 

 pulsation. For, indeed, when I consider the thickness, 

 the strength, and the nervous fibres of the dura mater ^ 

 I can imagine nothing else than that that membrane, 

 like all the others, is intended for the production of 

 motion. For it is probable that the dura mater 

 behaves as another diaphragm by the help of which 

 the brain draws in nitro-aerial spirits and in a sense 

 breathes. 



It is probable that the above described motion of 

 the meninges is natural, but yet that we can at will 

 increase its pulsation, just as we can that of the parts 

 which serve for respiration. And so by its diverse 

 motion various effects are produced in the bodies of 

 animals, as is probable. For according as that mem- 

 brane contracts itself more strongly or more weakly, 

 the nitro-aerial particles, i.e.^ the animal spirits, are 

 driven in greater or in less abundance into the brain 

 and thence into the nervous system. Hence if a 

 supply of animal spirits greater than usual is required 

 for carrying on more intensely the motive or the 



