282 The Older Doctrines [lect. 



succus nerveus, which he says "all recent authors admit is 

 " not a breath (flatus) or air, but has a liquid consistency like 

 "spirits of wine." This succus nerveus is agitated in two 

 directions along the nerves, from the periphery to the brain as 

 when it serves to generate sensations, and from the brain to 

 the muscles as when it serves to give rise to movements. Acting 

 in the latter direction it, as Willis also taught, performs a 

 double function; it not only causes visible movements by means 

 of the muscles, but also exercises a nutritive, plastic power by 

 which in all the tissues the crude material furnished by the 

 blood is fashioned into the living flesh. 



He thus expounds the mechanical contrivance by which the 

 spirituous juice (the succus nerveus) can, along the same nervous 

 channels, be made to act both from without inwards, and from 

 within outwards. 



" The nerve fibres are," he says, "by no means solid, full and 

 " impermeable, nor are they tubes hollow and empty like reeds, 

 " but are canals filled with a certain spongy material like elder- 

 " pith. Such a marrow of the fibres can easily be moistened by 

 "the spirituous juice of the brain, to which it is conjoined, and 

 " may indeed be saturated to turgescence, as we see sponges are 

 " saturated by water in contact with them." 



He uses as an illustration a sheep's intestine filled not with 

 water only, but with sponge impregnated with water, in which 

 a concussion at one end is in a moment communicated right to 

 the other end. 



"In the same way, if one of the extremities of the nerve 

 " fibre be compressed or pushed, or struck or pinched, forthwith 

 " the commotion and concussion or undulation ought to be com- 

 " municated right to the other end, because by reason of their 

 " contiguity, the parts lying first in an ordered series by pressing 

 " on those following, communicate the blow and the impulse 

 "right to the end. 



" Hence it follows that the fibres or spongy ducts of certain 

 "nerves turgid with the spirituous juice can be shaken or 

 " pinched by that gentle motion of the spirits by which the acts 

 " of the command of the will are in the brain carried out, and 

 " then, by concussing the whole length of the nerve through the 



