THE TISSUES 



49 



ary coil than the closure of the primary circuit, and therefore a 

 more powerful stimulation of the muscle (fig. 20). 





2. 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 anyone 

 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 mani- 

 fest its excitation by conducting the 

 impulse to the part of the muscle not in 

 the water and thus making it contract. 



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 contracted muscle, it may be 

 as dark as the dim band (fig. 21). 



These appearances may best be ex- 

 plained on the assumption that the fibrils 

 are the part of the fibre which shorten 

 and thicken, and that these fibrils chiefly 

 shorten where they are thickest in the dim band, 

 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 either band seems to be 

 indicated by the facts that they retain their reaction to polarised 

 light and staining reagents. 



Usually the contraction of a muscle occurs simultaneously 

 in all the fibres. This is because a nerve fibre passes to every 



4 



FIG. 21. Contraction of 

 Skeletal Muscle re- 

 laxed above, contracted 

 below. A is a diagram 

 of the cbange in a fibril ; 

 B shows the shading of 

 the clear band ; and C 

 shows the absence fof 

 auy alteration in the 

 influence of the two 

 bands on polarised 

 light. 



At the 



