5o8 SCIENCE PROGRESS 



something real descends the nerves to influence the muscles, 

 and so Borelli finally called this something the " succus 

 nerveus " or nerve-juice. The analogy he had in his mind was 

 that of an incompressible fluid in a flexible tube which can con- 

 duct rapidly from one end to the other of it the disturbance 

 produced by a tap or concussion. 



The position of the acute and critical Dane, Stensen or Steno 

 (1638-1686), was wholly agnostic : he wrote, "As the substance 

 of this fluid (nerve-juice) is unknown to us, so is its movement 

 undetermined." Although Steno left the problem of the nature 

 of nerve-impulses unsolved, yet he clearly distinguished between 

 neural activity and muscular irritability. 



The Englishman Thomas Willis (1621-1675) reverted to 

 Borelli's position, believing that spirits leapt from the nerves 

 into the muscle-fibres and so dilated them. 



Francis Glisson (1 579-1677), who formally introduced the 

 conception of irritability into physiology in 1662, contributed 

 something to this subject by showing experimentally that a 

 muscle did not alter in volume when it went into a state of 

 activity or contraction. By muscular " contraction," therefore, 

 we do not mean shrinking in volume ; the volume and the 

 density of a muscle remain constant whether in rest or in 

 action. 



The great investigator Stephen Hales (1677-1761) made an 

 interesting remark about the nerve-impulse, asking "whether 

 it is confined in channels within the nerves or acts along their 

 surfaces like electrical powers." This is probably the earliest 

 suggestion that the nerve-impulse and electricity have any- 

 thing in common. 



By many subsequent writers, nerve-impulses were considered 

 identical with electricity. The discoveries of Galvani seemed to 

 make such a thing probable. Those experiments of his known 

 as "contractions without metals" seemed to prove that muscles 

 would contract when stimulated by electricity of purely animal 

 origin. What, then, more probable than that nerve-impulses 

 and animal electricity were the same thing? Popular writers 

 forthwith assumed this to be the case, although it was not 

 warranted by any of Galvani's experiments. Galvani's experi- 

 ments really proved that the feeble differences of electrical 

 potential developed by injuring nerves or, for instance, by the 

 activity of the heart, were sufficient to make a muscle (of the 



