92 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



condition, the muscle-substance appears marked by very 

 regular transverse bands, which are alternately opaque 

 and transparent ; and it is characteristic of the group of 

 animals to which the crayfish belongs that their muscle- 

 substance has this striped character in all parts of the 

 body. 



A greater or less number of these fibres, united into one 

 or more bundles, constitutes a muscle ; and, except when 

 these muscles surround a cavity, they are fixed at each 

 end to the hard parts of the skeleton. The attachment 

 is frequently effected by the intermediation of a dense, 

 fibrous, often chitinous, substance, which constitutes the 

 tendon (fig. 19, A ; t) of the muscle. 



The property of the living muscle, wliich enables it to 

 be the cause of motion, is this : Every muscular fibre is 

 capable of suddenly changing its dimensions, in such a 

 manner that it shortens and becomes proportionately 

 thicker. Hence the absolute bulk of the fibre remains 

 practically unchanged. From this circumstance, muscular 

 contraction, as the change of form of a muscle is called, 

 is radically different from the process which commonly 

 goes by the same name in other things, and which 

 involves a diminution of bulk. 



The contraction of muscle takes place with great force, 

 and, of course, if the parts to which its ends are fixed 

 are both free to move, they are brought nearer at the 

 moment of contraction : if one only is free to move that 

 is approximated to the fixed part ; and if the muscular 



