236 PHYSIOLOGY 



globule. Thus, if the globule be in sulphuric acid it undergoes a certain 

 amount of polarisation, and becomes positively charged. By altering the 

 charge of such a globule we can change its shape, as is shown diagram- 

 ma tically in Fig. 91. If B represents the shape of the globule lying on the 

 plate in some weak sulphuric acid, A will represent the shape of the globule 

 when it is connected with the negative pole of a battery, while c will repre- 



sent its shape when it is connected 

 ** with the positive pole of a battery, 



the other pole in each case being 



connected with the acid. If we 

 91. consider muscle as made up of a 



series of chains of oval particles, a 



chemical change in the surface of these particles, causing an increase of 

 surface tension, will tend to make them assume the globular shape, and 

 will therefore cause a shortening and thickening of the whole fibre. 



According to Schafer, contraction is associated with a flow of the outer hyaline 

 contents of the sarcous element into the tubular structure forming the middle portion. 

 Such a flow may be determined either by osmotic differences between the centre and 

 periphery of the sarcous element, or by a change in the surface tension obtaininsJ^tween 

 the isotropic fluid at the ends and the anisotropic structures in the centre of tJ^Pnuscle 

 prism. 



The tendency of recent investigation is all in favour of the second hypo- 

 thesis, namely, that the essential factor in the processes of excitation and 

 contraction is an alteration of surface. In the first place the electrical 

 changes accompanying the excitatory process denote a polarisation or 

 accumulation of ions on the surfaces situated in the excited area. The 

 chemical change which is responsible for the current of action, or the negative 

 charge at the excited spot, takes place almost instantaneously and disappears 

 somewhat more slowly. It would seem that the excitatory process consists 

 essentially in the setting free of certain ions on the surface or surfaces in 

 the contractile tissue, and that the passing away of the excitatory state 

 is due to the disappearance of these ions, either by diffusion away into the 

 surrounding fluid or by further chemical changes, such as oxidation. A 

 study of the development of tension and of heat production in a muscle on 

 excitation has shown that in both cases the yield of energy on excitation 

 is increased by lengthening and diminished by shortening the muscle. Now 

 alteration in length of the muscle will not alter its volume, but will alter 

 the extent of its longitudinal surfaces, and it appears therefore that the 

 production of heat as well as of mechanical energy is not a volume, but a 

 surface effect. Finally the work of A. V. Hill on the heat production in 

 muscle seems to show that the rise of tension in a muscle on excitation is 

 due to the liberation of chemical bodies, of which lactic acid is certainly one, 

 in the neighbourhood of certain longitudinal surfaces or membranes, and 

 that the presence of these bodies changes the tension at such surfaces and 

 thereby the longitudinal tension of the fibre. The extent and intensity of 

 the production of these bodies must depend on the area of the chemically 



