iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 255 



It is hardly necessary to mention the grossly mechanical con- 

 ception of the early physicians, who compared the influence of the 

 nerves upon the muscles to the pulling of bell wires in order 

 to ring them. 



Another hypothesis that now seems little less puerile, although 

 under various forms it prevailed for centuries, was that by which 

 the nerves were regarded as hollow tubes or canals, within which 

 circulates a fluid, or a more or less ethereal and mystical gas, that 

 conveys the movements ordered by the brain and the ' sensations 

 from the sense organs, and received various names according to 

 the epoch and school of thought (spiritus vitalis or animalis, 

 pneuma, fluidum nerveum, etc.). The paralysis produced by 

 ligation of a nerve was explained as the necessary effect of the 

 arrest of the fluid that circulates in the nerve tubes. 



At a much later time physicians conceived the conduction of 

 neural activity as a phenomenon analogous to the undulatory 

 transmission of a mechanical impulse through an elastic medium, 

 the nerves being regarded either as vibrating cords or as being 

 composed of a number of minute elastic particles which transmit 

 their oscillations one to another (Robinson, 1630). The theory of 

 an imponderable nervous fluid was, however, more plausible and 

 found more favour. Especially after the phenomena of frictional 

 electricity and the laws of its propagation became known many 

 physicians thought they could compare activity in the nerve to 

 that of an electrical apparatus. Hausen (1743) and de Sauvages 

 (1744) were the first who upheld the electrical nature of nervous 

 activity. Haller criticised this hypothesis, holding it to be 

 unfounded and contradicted by two important experimental facts 

 absence of insulation of the nerves, and the paralysing effects of 

 tying the nerve. 



It was not till Walsh (1773) had pointed out the electrical 

 nature of Torpedo shocks, and Galvani had discovered animal 

 electricity, that the hypothesis of the electrical nature of nervous 

 activity became more widely known and accepted, and it has 

 only acquired the definite position of a scientific theory within 

 recent years. 



The hypothesis of absolute identity between electricity and 

 neural activity received a fatal blow when Helmholtz (1850) 

 demonstrated by exact physical methods that conduction in the 

 nerve proceeds at an incomparably slower rate than electrical con- 

 ductivity (see p. 203). Nevertheless it appears highly probable 

 from the work on animal electricity done by Nobili, Matteucci, 

 and particularly by Du Bois-Reymond on the negative variation 

 of the nerve current and the phenomena of electrotonus (1843), 

 that electrical energy does play a part in nervous conduction, 

 although under a different form from that assumed in the theory 

 of their identity. 



