S6 SCIENCE PROGRESS 



bility and the conductivity of nerve may be shown to be two 

 distinct and separable processes, the former due to a local 

 change at the seat of excitation, the latter to a wave propagated 

 with a finite and measurable speed ; the continued normal 

 behaviour of a muscle or nerve has been shown to depend upon 

 a precise adjustment of the hydrogen-ion concentration and 

 an accurate balance between various metallic ions in the fluids 

 bathing it ; the change of form produced by contraction has 

 been demonstrated to be associated with the liberation of lactic 

 acid from glucose in the interior of the muscle cell : all these 

 phenomena probably have their counterparts in other tissues. 

 Thus, apart from their intrinsic interest, the investigation of 

 these two very important tissues is likely to lead us to reliable 

 conclusions as to the behaviour of living cells in general. Nearly 

 all living cells show the phenomena of excitability — these cells 

 show it in a highly specific form. We know as yet very little 

 about the mechanism by which excitation occurs, but it would 

 appear that in some sense all forms of excitation are really 

 electrical, or ionic, in nature, and set up a propagated dis- 

 turbance, whose only known accompaniment is an alteration 

 in ionic permeability associated with the electrical change, 

 and due possibly to some kind of transmitted upset of the 

 orientation of the protein molecules of the nerve sheath. 

 Various characteristics of the excitatory process and of the 

 propagated disturbance are known — characteristics established 

 by careful and often very beautiful physical experiments — but 

 there is little doubt that much still remains to be done, simply 

 in giving a clear description — apart from any theory — of the 

 actual physical facts accompanying them. The welding, co- 

 ordinating, and modifying influences of the central nervous 

 system on such impulses must obviously remain a mystery 

 so long as the actual physical accompaniments of the impulses 

 are not known. The physical theory of the nature of the 

 impulse will probably require considerable improvements in 

 knowledge, both of molecular physics and of colloidal chemistry, 

 for its description. 



More is known about the physical accompaniments of 

 the activity of a muscle cell. It shows the same phenomena 

 of excitability as a nerve, but in addition to these certain 

 chemical, physical, and mechanical changes occur whose in- 

 vestigation has given us, partly by analogy, an extensive 

 insight into the nature of the internal mechanism of the cell. 

 The muscle twitch, the fundamental unit on which all muscular 

 action is based, consists of a temporary change in the elastic 

 properties of the fibre, accompanied (if the fibre be allowed to 

 shorten) by the production of mechanical energy, and by the 

 evolution of heat, the using up of oxygen, and the production 



