282 THE BODY AT WORK 



with an electric current. Its relatively rapid progress, on the 

 other hand, equally excludes the hypothesis that it is a move- 

 ment of ions, as that phenomenon is observed in solutions of 

 salts. 



What is the nature of the process by which energy is con- 

 veyed along a nerve ? When speaking of the passage of 

 impulses from receptors to the central nervous system, and 

 through this to effectors, we have used the vague expression 

 " molecular change," to avoid the necessity of being more 

 precise. But the problem is of such profound interest that we 

 look with eagerness for any hint of the direction from which 

 light will eventually be thrown upon it. Recent discoveries 

 regarding the nature of electricity, combined with investiga- 

 tions at present in progress as to the physical constitution of 

 proteid substances, give more than a hint. Hitherto the choice 

 has lain between a chemical and a physical explanation ; now 

 the border-line between chemistry and physics, always waver- 

 ing, has disappeared. The hypothesis that an impulse is a 

 progression of chemical change has meant in the past that the 

 " wave " was due to the oxidation of substances contained in 

 nerve, with liberation of C0 2 and H 2 0. Various considerations 

 render such metabolism of the substance of which nerve-fibres 

 are composed improbable. In the first place, nerve-cell 

 bodies contain a store of material, tigroids (p. 320), which is 

 recognizably drawn upon during nervous activity. It would 

 appear, therefore, to be the tigroids, and not the substance of 

 the nerve-fibre, which supply the energy transmitted along a 

 nerve. Then, again, the axon of a nerve-fibre, enclosed as it 

 is in a tube of fat, is peculiarly ill-placed for the reception of the 

 nourishment which would be needed to make up for waste, if 

 its metabolism be fluctuating and at times excessive. Nor have 

 nerves more than a very meagre blood-supply. Secondly, 

 observation does not give any support to the hypothesis of 

 fluctuating metabolism. A nerve does not give off more C0 2 

 when active than when passive. Nor does it become acid. 

 Thirdly, nerves, or, to be quite accurate, medullated nerves, 

 are indefatigable. Their capacity for conduction is not 

 diminished by previous use, as it would be were it dependent 

 upon their reserve of nutriment. These various considerations 

 rule out a " chemical " explanation of the old-fashioned type. 



