i GENEEAL PHYSIOLOGY OF MUSCLE 85 



part of the great protein molecule are so close together that a very 

 weak stimulus suffices to bring about an explosion, in which most 

 of the oxygen atoms combine with atoms of carbon and hydrogen 

 to form carbonic acid and water. It is more difficult to explain 

 why the explosion is confined to a small part of the explosive 

 mass instead of discharging it completely, as in the case of a 

 loaded fire-arm. But the universally accepted principle is that 

 the potential chemical energy of the muscle substance is the primary 

 source of muscular energy in all its manifestations. 



How does the explosive reaction of the muscle produce its 

 shortening or elastic tension ? The answers to this question are 

 by no means unanimous, and physiologists differ, according as the 

 one or the other sign of muscular activity receives the more con- 

 sideration from them. 



We need not discuss the earlier hypotheses which are collected 

 in the classical text-books of Haller (1792) and Johannes Miiller 

 (1844), but may confine ourselves to the later and more probable 

 theories, commencing with that of E. Weber. I 



Schwann had already suggested that muscle acts by elastic 

 forces, but Weber was the first to clear up the obscurity that 

 prevailed as to contractility and elasticity in his classic work on 

 Muskelphysik, published 1846. According to the theory formulated 

 by Fontana elasticity is an inherent physical property which tends 

 to preserve the natural form of the muscle, and thus acts in the 

 contrary sense to contractility, i.e. it limits the contraction and 

 brings the muscle back to equilibrium as soon as the active state 

 ceases. But Weber pointed out that the natural form of the 

 muscle which depends on its elastic equilibrium is not constant, 

 but varies freely with the external and internal conditions of the 

 life of the muscle. As a metal rod lengthens when heated, and 

 shortens again on cooling, because different degrees of temperature 

 alter the equilibrium of its atoms and produce a reaction of its 

 elastic forces which expand in the first case and contract in the 

 second, so the molecular arrangement differs in the muscle accord- 

 ing as it is at rest or excited, and its external form differs 

 accordingly. The active muscle is short and thick, the inactive 

 long and thin, and in suddenly passing from one state to the 

 other the muscle contracts or expands, not against the elasticity, 

 but by an elastic reaction in order to assume the natural form of 

 equilibrium which corresponds to its active or inactive state. On 

 Weber's view the extension of a muscle by a weight is not com- 

 parable with the short ening due to a stimulus : the weight stretches 

 the muscle against its elastic forces ; the stimulus, on the contrary, 

 causes a sudden alteration in its chemical equilibrium, and therefore 

 in the elastic forces, which are the immediate cause of contraction. 



Weber was the first who submitted the elasticity of muscle, 

 and the changes it undergoes in various conditions, to strict 



