MUSCLE AS THE SOURCE OF MOTION. 



93 



fibre surrounds a cavity, tlie cavity is lessened when the 

 muscle contracts. This is the whole source of motor 

 power in the crayfish machine. The results produced 

 b}' the exertion of that power depend upon the manner 



Fig. 20. — Astacu.sfliiviatUin.— The chela of the forceps, with one side 

 cut away to show, in A, the muscles, in B, the tendons (x 2). 

 cj}, carpopodite ; 2^>'2>, propodite ; r/y^, dactylopodite ; m, adductor 

 muscle ; m', abductor muscle ; t, tendon of adductor muscle ; f, 

 tendon of abductor muscle ; u\ hinge. 



in which the parts to which the muscles are attached 

 are connected with one another. 



One example of this has already been given in the 

 curious mechanism of the gastric mill. Another may be 

 found in the chela which terminates the forceps. If the 



