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, which 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 
