170 LOCOMOTION. [CHAP. vn. 



of which the fulcrum is for the most part in a joint, i. e. at one 

 extremity, the resistance at the further end, and the power (or the 

 muscle) in the intermediate portion. In most cases the muscles 

 are attached very near the fulcrum, as in the familiar instance of 

 the biceps, inserted into the tubercle of the radius. By this dis- 

 position, a contraction of a single inch in the muscle moves the 

 hand in the same time through the extent of a foot, but then 

 the hand moves through every inch with only the twelfth part of 

 the power exerted in the muscle ; *. e. a resistance at the hand equal 

 to one-twelfth of the force of the muscle would stop its motion. 

 Thus, force is converted into extent and velocity of movement, at 

 the same time that the great inconvenience is avoided of having the 

 muscles extended like bow-strings between the distant ends of 

 jointed continuous levers. By the junction of two or more levers 

 in one direction, as in the different segments of the extremities, 

 the extent and velocity of their united actions are given to the 

 extreme one. A blow of the fist may be made to include the force 

 of all the muscles engaged in extending the shoulder, elbow, and 

 wrist. 



In the conspicuous example of the tendo Achillis, inserted into 

 the os calcis, the resistance (or the weight of the body) rests on the 

 astragalus, intermediate between the power and the fulcrum, which 

 is here the ground, pressed by the ball of the foot. The extent 

 and velocity of motion are here converted into power. If the tendo 

 Achillis draws on the heel when the foot is off the ground, the front 

 part of the foot is extended on the lower end of the tibia as a 

 fulcrum, and exhibits an instance of the other of the three varieties 

 of lever. 



Of the Function of Muscles. The great property of muscular 

 tissue is that of shortening in a particular direction, and this pro- 

 perty is called contractility. It is not that mechanical power which 

 elastic substances possess of shortening themselves on the removal 

 of some force which has stretched them, but it is an endowment, 

 responsive to appropriate stimuli, and diminishing or disappearing 

 with the healthy state of the tissue (p. 57). 



The distinction between the contractility, the elasticity, and the 

 physical tenacity of a muscle may be illustrated by the following 

 imaginary experiment : Suppose the leg of an animal so severed 

 from the trunk as to hang by a single muscle, which, after re- 

 taining its contractility for some time, were gradually to lose it. 

 The limb would at first be borne up by the contractile power ; 



