NERVOUS ACTIVITY 507 



For the sake of clearness it may be well to say at once that 

 muscles have irritability of their own, after all their nerves are 

 cut, but that unless nerve-impulses (tonic) are constantly 

 pouring down upon them, and unless stimuli to action are 

 frequently being received by them, they will waste away because 

 there is nothing to call forth the power of contraction which 

 they do possess. 



As regards views on the working of the nerves, we find 

 nothing of any consequence from the death of Galen (200 a.d.) 

 to the time of Vesalius (1543), for the interval of more than 

 a thousand years was occupied by the Dark Ages when there 

 was hardly any investigation of living nature, and very little 

 curiosity about the mysteries of life. 



Vesalius wrote of muscle that it " also receives branches of 

 arteries, veins, and nerves, and by reason of the presence of the 

 nerve is never destitute of animal spirits so long as the 

 animal is sound and well .... Nor do I with Plato and 

 Aristotle (who do not at all understand the nature of 

 muscle) attribute to the flesh so slight a duty as to serve 

 the purpose of lessening the effects of heat in summer and 

 of cold in winter. On the contrary, I am persuaded that the 

 flesh of muscles, which is different from everything else in the 

 whole body, is the chief agent by the aid of which (the nerves, 

 the messengers of the animal spirits, not being wanting) the 

 muscle becomes thicker, shortens and gathers itself together." 

 Thus writes Vesalius, who does not attempt any explanation : 

 he does not know what spirits are, or how they affect the 

 muscle, or why it shortens when they do affect it ; he only knows 

 that something in nerves does influence muscle. 



G. A. Borelli of the University of Pisa (1608-1679) the 

 mathematician and author of the De motu animalium, en- 

 deavoured to be more exact in his conception of how this 

 activity of muscle came about under the influence of nerve- 

 impulses. 



Borelli at the outset fell into the error that a muscle 

 increases in volume when it goes into activity. He then 

 attempted to get some idea of what these animal spirits were 

 which apparently could inflate muscle, and he thought they 

 must resemble air. But when he cut an active muscle across 

 under water no bubbles of air or gas come out of it ; therefore, 

 he concluded, the spirits were not gaseous. Nevertheless, 



