7 i8 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



Changes during Contraction Theories of Contraction. In contractions, 

 according to Schafer, the clear substance between Krause's membrane 

 and the sarcous element passes into the canals, which are open towards 

 Krause's membrane, but closed towards Hensen's line. The sarcous 

 element therefore swells up, and the sarcomere is shortened. In the 

 extended muscle the clear substance leaves the pores of the sarcous 

 element, and accumulates in the space between it and Krause's mem- 

 brane. The sarcomere is thus lengthened and narrowed. While the 

 existence of Schafer's pores is not admitted by all observers, there is a 

 pretty general agreement that the sarcomere, like the cytoplasm of an 

 amceboid cell, does consist of two substances, one of which (the hyalo- 

 plasm of the cell, the clear material of the sarcomere) interpenetrates 

 the other (spongioplasm of the cell, substance of the sarcous element) ; 

 and that in relaxation the clear fluid .passes from the sarcous element 

 to the ends of the sarcomeres, whereas in contraction it passes in the 

 reverse direction into the sarcous elements. Whether the fluid passes 

 into and out of the meshes of an actual network, or along actual physical 

 pores in the sarcous element, or, whether it is transferred by some 

 process like molecular imbibition (p. 420), need not be discussed here, 

 since it is not definitely known. The fundamental question by what 

 process the transference is determined when the muscle is excited also 

 remains unsettled. So far as is known at present, it is probable that 

 the mechanical energy of the contracting muscle must be derived from 

 the transformation of chemical energy into one of three forms : energy 

 associated with osmotic processes, energy associated with imbibition, 

 and energy associated with changes of surface tension. It is not diffi- 

 cult to see that a sudden increase in the osmotic concentration in the 

 sarcous element, due to the breaking up of large molecules or colloid 

 aggregates into small molecules, or the liberation of electrolytes from 

 the colloids, might lead to the rapid passage of water into it from 

 the bright bands. A sudden change of permeability of the sarcous 

 elements for dissolved substances in the clear fluid would have a 

 similar effect. The same is true of a change in their power of imbibi- 

 tion. But, according to Bernstein, it is scarcely to be supposed that 

 the extraordinarily rapid movement of water molecules which must 

 occur in contraction can be accounted for either by osmosis or by im- 

 bibition. A more plausible theory is that the surface tension say 

 between the substance of the sarcous element and the clear fluid is 

 altered. That the shortening of the muscle in fatigue (p. 723) and 

 rigor (p. 748), as well as its shortening in normal contraction, is due in 

 some way to the liberation of metabolic products, especially lactic acid, 

 is a theory of some standing, and fresh evidence in its favour has been 

 recently supplied. Thus it has been pointed out that the course of 

 heat production in the active muscle, and its relation to the time of 

 the mechanical response, and the development and time relations of 

 the electrical change which precedes that response, can be very naturally 

 explained on the supposition that the liberation of lactic acid on or 

 near some surface in the contractile substance is an essential factor in 

 the contraction (Mines, etc.). It is known that in the presence of acid 

 on the surface of certain colloid structures shortening occurs (Fischer 

 and Stnetman). 



The substance of the sarcous element which forms the dark stripe 

 is doubly refracting, and therefore rotates the plane of polarization, 

 but the clear substance of the light stripe is singly refracting. When 

 an uncontracted fibre is viewed with crossed nicols, the dim stripe 

 accordingly appears bright in the otherwise dark field. In the con- 

 tracted fibre the doubly refractive material remains in the stripe which 



