THE TISSUES 51 



and, finally, at the cathode on closing, disappear. When 

 the muscle is in one stage of the degeneration which follows 

 separation from its nerve, the anodal closing contraction 

 (Fig. 19, Anode M) becomes much exaggerated. This is 

 called the reaction of degeneration. 



3. THE CHANGES IN MUSCLE DURING CONTRACTION. 

 I. Change in Shape. 



The most manifest change is an alteration in the shape 

 of the muscle. It becomes shorter and thicker. This 

 any one can see by studying their own biceps muscle. 

 Contraction of muscle, however, is not a necessary result 

 of excitation. Thus a part of a 

 muscle when dipped in water may 

 fail to contract when stimulated, but 

 may manifest its excitation by caus- 

 ing the part of the muscle not in the 

 water to contract by conducting. 



In skeletal muscle the shortening 

 and thickening of the muscle as a 

 whole is due to the shortening and 

 thickening of the individual fibres 

 and their fibrils. 



In these fibrils the shortening and 

 thickening is most marked in the 

 dim band. The clear band also 

 shortens, and at the same time it 

 becomes darker till, in the fully con- 

 tracted muscle, it may be as dark as 

 the dim band. 



These appearances may best be explained on the assump- 

 tion that the fibrils are the part of the fibre which shorten and 

 thicken, possibly by a flow of fluid into them, and that these 



fibrils chiefly shorten where they are thickest in the dim 



band. At the same time by the contraction of the fibrils in 

 the clear band, adjacent dim bands may be supposed to be 

 pulled nearer to one another, and to cast a shadow over the 

 clear band. That no actual chemical change takes place in 



FIG. 20. Contraction of 

 Skeletal Muscle relaxed 

 above, contracted below. 

 A is a diagram of the 

 change in a fibril ; B shows 

 the shading of the clear 

 band ; and C shows the 

 absence of any alteration 

 in the influence of the two 

 bands on polarised light. 



