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 {93} fibre surrounds a cavity, the cavity is lessened when the muscle contracts. This is the whole source of motor power in the crayfish machine. The results produced by the exertion of that power depend upon the manner in which the parts to which the muscles are attached are connected with one another. [Illustration: FIG. 20.—_Astacus fluviatilis._—The chela of the forceps, with one side cut away to show, in A, the muscles, in B, the tendons (× 2). _cp_, carpopodite; _prp_, propodite; _dp_, dactylopodite; _m_, adductor muscle; _m′_, abductor muscle; _t_, tendon of adductor muscle; _t′_, tendon of abductor muscle; _x_, hinge.] 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 {94} articulation of the last joint (fig. 20, _dp_) with the one which precedes it (_prp_) is examined, it will be found that the base of the terminal segment (_dp_) turns on two hinges (_x_), formed by the hard exoskeleton and situated at opposite points of the diameter of the base, on the penultimate segment; and these hinges are so disposed that the last joint can be moved only in one plane, to or from the produced angle of the penultimate segment (_prp_), which forms the fixed claw of the chela. Between the hinges, on both the inner and the outer sides of the articulation, the exoskeleton is soft and flexible, and allows the terminal segment to play easily through a certain arc. It is by this arrangement that the direction and the extent of the motion of the free claw of the chela are determined. The source of the motion lies in the muscles which occupy the interior of the enlarged penultimate segment of the limb. Two muscles, one of very great size (_m_), the other smaller (_m′_), are fastened by one end to the exoskeleton of this segment. The fibres of the larger muscle converge to be fixed into the two sides of a long flat process of the chitinous cuticula, on the inner side of the base of the terminal segment, which serves as a tendon (_t_); while those of the smaller muscle are similarly attached to a like process which proceeds from the outer side of the