84 PHYSIOLOGY CHAP. 



According to Fano the reversal of the diphasic curve proves 

 that it does not depend solely, as is usually assumed, on a wave of 

 negativity spreading from the near points to those more remote 

 from the stimulus, but is due to a wave of positivity immediately 

 following the former. In the normal electrical tracing, too, the 

 relations between the two phases, negative and positive, vary 

 the first predominating in some cases, the second in others. It is 

 not improbable that these different types of the electrical curve 

 depend on the relations between the katabolic and anabolic 

 processes of cardiac muscle, and that stimulation of the vagus 

 exaggerates the latter. 



XII. The innumerable physical, chemical, and histological 

 researches on muscle which have thus been briefly summarised 

 have yielded an extraordinary wealth of physiological data, from 

 which some solution of the difficult problem of the origin of 

 muscular force may be constructed some hypothesis able to 

 explain the internal mechanism on which the contraction and 

 relaxation of the muscle depends, or more generally, its capacity 

 for passing suddenly from the state of comparative rest to that of 

 activity, and vice versa. 



An exhaustive theory of the mechanism of muscular excita- 

 bility must cover a series of difficult problems, among which are 

 the following: 



(a) How is the excitation of the nerve end-plate transmitted 

 to the muscle fibre ? 



(6) On what does the sudden contraction (isotonic) and elastic 

 tension (isometric) depend ? 



(c) What process gives rise to the sudden relaxation of the 

 muscle, i.e. the cessation of the elastic tension on which shortening 

 depends and the production of the elastic tension to which lengthen- 

 ing is due ? 



(d) How are the excitatory impulse and the wave of contraction 

 and relaxation conducted along the muscle fibre ? 



Speaking generally, it may be said that these and other 

 problems have at present received no proper scientific solution, 

 so we must confine ourselves to a critical investigation of the 

 principal hypotheses that have been put forward. 



It is now universally agreed that the physiological combustion 

 of certain chemical constituents of the tissue which are bound up 

 with the protein molecule or intimately connected with it, are the 

 prime source of muscular energy, and that the transformation of 

 potential chemical energy into mechanical energy, either in the 

 form of elastic tension or in that of external work, is performed 

 according to the law of the conservation of energy. There is a 

 general tendency to consider the chemical state of the resting 

 muscle substance as one of unstable equilibrium, in which the 

 atoms of oxygen and the groups of combustible atoms which form 



