THE CHEMISTRY OF MUSCLE, 73 



that the immediate cause of the shortening of the muscle is to be found in 

 changes of surface tension or osmotic pressure. The instrument or machine 

 acted upon is the fibril which is regarded as a coherent gel structure embedded 

 in a more liquid material, the sarcoplasm. As each fibril consists of a linear 

 series of similar sarcomeres attention is directed to the reaction as it occurs in 

 this unit. According to one set of theories the energy liberated by the chemical 

 reaction alters in some way the surface-tension relations within the sarcomere. 

 It might be supposed, for example, that a change occurs at the interface 

 between the dim and light bands. If the surface tension in the material of the 

 dim band is increased at this surface the effect would be to round off this 

 surface, thus causing a shortening in one diameter with a corresponding 

 increase in the other.* Another suggestion t is that the change in surface 

 tension affects the submicroscopic particles of the fibrils, especially in the 

 dim bands where they are densest, with the result that they are drawn together 

 into lafger aggregates, some of the intervening water molecules being forced 

 into the area of the light bands. An increasing number of workers, however, 

 have inclined to the view that the mechanism of the contraction is to be found 

 in a change in the osmotic properties or power of imbibition of the material 

 within the sarcomere, especially the material of the dim band.f As explained 

 above, the results of recent work upon the muscle point to lactic acid as the 

 immediate cause or exciting agent of the shortening. Some of the advocates 

 of the imbibition theory have suggested therefore that it is the lactic acid, 

 formed within the sarcomere or the adjacent sarcoplasm as the result of a 

 stimulus, which acts upon the anisotropic material and increases its osmotic 

 pressure. Warrant is found for this hypothesis in the well-known fact that the 

 osmotic pressure of solutions of organic colloids is increased by the action, of 

 acids. It may be assumed that the taking up of more water by the con- 

 stituent particles of the anisotropic material would tend to give them a more 

 spherical shape and would thus account for the shortening. Or if one assumed 

 that the Krause membrane is a real membrane, shutting off the sarcomere 

 at each end, then a flow of water might take place from the sarcoplasm into 

 the sarcomere, causing the latter to round off and shorten in its long diameter. 

 On this kind of theory the relaxation is explained as being due to the removal 

 of the acid by oxidation or otherwise. The theory has many variations as 

 regards details, but the central idea is that the lactic acid is the agent which 

 initiates the contraction by increasing the imbibition power of some substance 

 in the fibril. The shortening in rigor mortis is explicable in the same way 

 as an acid effect. 



The Effects of Exercise on the Condition of the Muscle. — It 



is known that when a muscle is not used it atrophies. On the 

 contrary, exercise leads to an increase in the size, strength, and tone 

 of the muscle. If the motor nerve is cut so that the muscle is 

 completely paralyzed a true degenerative atrophy ensues which 

 results finally in a disappearance of the muscular tissue. Atrophy, 

 so-called simple atrophy, occurs also from mere disuse. In bed- 

 ridden patients or in muscles prevented from contracting by 

 splints and bandages an obvious decrease in size and strength may 

 be observed. The ultimate cause of these changes is to be found, 

 no doubt, in an alteration in the character or intensity of the normal 



* Macallum, "Surface Tension and Vital Phenomena," University of 

 Toronto Studies, Physiological Series, No. 8, 1912. 



t Lillie, "Science," Aug. 2.3, 1912. 



t Engelmann, loc. cit.; McDougall, "Journal of Anatomy and Phvsiologv," 

 31, 410, 1897, and 32, 187, 1898; Meigs, "American Journal of Physiology," 26, 

 191, 1910, and 22, 477, 1908; Pauli, "KoUoid-chemie der Muskelkontraktion," 

 1912. 



