and Plants to the Electrical Phenomena associated with it. 39 



When a muscle is roused to activity by the presence of an exciting 

 cause, its mechanical properties suddenly change. It shortens and, if 

 the shortening is resisted, becomes tight. If the resistance is such as 

 cannot be overcome, it tightens without shortening. With reference 

 to this mechanical change, we know that it is dependent on chemical 

 change, and that that change is oxidation. Admitting these proposi- 

 tions, we must necessarily believe that the oxidation is sudden, i.e., 

 explosive, because its mechanical effect (the tension or tautness I have 

 mentioned) attains its maximum at a very short period after the 

 moment at which the process begins. 



At ordinary temperature we find that in a whole muscle the tension 

 which is induced by an excitation attains its maximum in about 3/100 

 second. But if we fix our attention on a single muscular element, 

 i.e., on one of the infinite number of molecular mechanisms by the 

 cooperation of which the mechanical change consequent on excitation 

 is brought about, it can be shown that in each taken separately a much 

 shorter duration than 3/100 second must be assigned to the process of 

 transition. According to Bernstein, less than 1/100 second must be 

 assigned to the chemical process which takes place in a muscular 

 element in response to a single stimulus.* 



This chemical process of extreme suddenness is followed without 

 measurable loss of time by the conversion of the chemical energy of 

 the oxidisable material into mechanical energy, which immediately 

 manifests itself either in shortening or in the effort to shorten. The 

 way in which this transference takes place must for the present be 

 left an open question, for, as Professor Engelmann explained in the 

 Croonian Lecture for 1895, the transformation of chemical into 

 mechanical energy consequent on excitation of muscle, though by no 

 means an insoluble, is still an unsolved physical problem. We know 

 how much chemical energy is liberated, we know how much work is 

 done, and how much heat is wasted, but we cannot explain how it 

 happens ; it being difficult to suppose that the temperature required for 

 such transformation can exist in living muscle. 



The absence of a sufficient physical theory of the origin of muscular 

 force does not, however, deprive the mechanical manifestations of the 

 process of their value as simple, measurable, and controllable indica- 

 tions of functional activity. Whether we take the case in which a 



* See Pfluger's 'Archiv,' vol. 67, p. 211, 1897, where Bernstein describes his 

 method of measuring the period of latency. As in the method described by me in 

 the 'Journal of Physiology' in 1895, a magnified image of the moving surface of a 

 muscle excited directly is received on a slit, behind which a sensitive plate passes. 

 From the curve so obtained, Bernstein determines the moment at which the rate of 

 expansion increases most rapidly, and regards this as the moment at which the 

 moving force is at its maximum. This conclusion, of course, applies to the part of 

 the muscle immediately excited. 



