16 ZOOLOGY 



seems to be of a fluid, or at least a semi-fluid character, and the 

 act of contraction seems to be accompanied by an increase in 

 volume in this substance, for the fibre in shortening its length 

 increases, as we have seen, its diameter and gains in volume 

 on the whole a fact familiar to every schoolboy who " raises " 

 his biceps muscle. In the most highly developed contracting 

 fibres, such as are found in the flesh of our own bodies and that of 

 the highest animals, we meet with a phenomenon known as the 

 cross-striping of the fibre. Every contractile fibre when at 

 rest is found to be crossed by a series of dark plates, placed at 

 regular intervals from one another like the sleepers on a railway 

 line. Between these plates the fibre is made up of clear semi- 

 fluid substance. When contraction occurs the clear substance 

 increases in quantity, and appears to be imbibed by the dark 



FIG. 2. Part of a cross-striped muscle fibre: aw, 

 anisotropic substance ; is, isotropic substance in a 

 single fibril ; /, a fibril ; n, nuclei. 



plates which swell and at the same time become less dark in 

 colour. The dark plates are known by their behaviour to- 

 wards light to be solid, and their purpose seems to be by 

 exercising an attraction on the fluid substance to increase the 

 rapidity of the contraction. It is known that cross-striped 

 fibres contract much more rapidly than plain ones, and the 

 striping is best marked in the fibres which move the wings of 

 insects ; and these muscles must in many cases contract and 

 relax many hundreds of times in a second. 



The increase in volume then of the semi-fluid substances 

 seems to be a proximate cause, if not the proximate cause, in 

 producing contraction. Now the only way in which we can 

 conceive of this increase in volume being brought about is by 

 chemical decomposition. When a match is applied to gun- 

 powder contained in a gun-barrel an enormous increase in 

 volume takes place, owing to the fact that the materials of the 



