ADJUNCTS OF THE MUSCULAR SYSTEM. 103 



of the os calcis, and of the astragalus (forming a single bone 

 in movements of this kind), at the level of the tibio-tarsal 

 joint, and, consequently, between the fixed point and the 

 point of application of the power. The lever arm of the 

 power is thus longer than the arm of the resistance ; and, 

 consequently, the power to raise the body, displayed by the 

 muscles of the calf, may be inferior to the weight of the 

 body itself, as is shown us by the law of levers of the second 

 kind (Fig. 29). 



The lever of the third kind (power between fulcrum and 

 weight) is the most common ; it is the lever of locomotion par 

 excellence: we find it in most partial or complex movements, 

 and especially in the movements of flexion and extension. We 

 need not examine the shoulder and elbow joints in the act 

 of prehension, in order to establish the type of this lever, for 

 here the arm of the power is always snorter than that of 

 the resistance, so that the energy of the muscular contrac- 

 tion must always be superior to the resistance to be 

 overcome. Yet, on the other hand, the distance traversed 

 by the resisting extremity of the lever 

 (the hand, for example, in the flexion 

 of the forearm) is greater than that 

 traversed by the point of application 

 of the force (insertion of the biceps 

 in the upper part of the forearm) ; 

 and thus what is lost in power is 

 gained in extent. Fi 



The working of these different elbow, as a" b iever of the 

 levers is facilitated by the disposi- 

 tion of the bones, in which a deep cavity is hollowed out 

 (cancellated texture), filled with a soft and almost liquid 

 matter (marrow). By this arrangement the weight of the 

 bony levers is diminished, while the bone affords a surface 

 sufficiently large for the insertion of the numerous muscles 

 by which it is moved. The substance which fills these cavi- 

 ties is the lightest in the whole system, fat (marrow, in the 

 adult). Finally, this arrangement of the bones is favorable 

 to the part which they play as supports, for mechanics teach 

 us that of two columns of the same height, and formed of 

 an equal quantity of matter, one being solid and the other 

 hollow, the latter will be the stronger. This principle ap- 



* 0,A, Humerus. A, 0'. Forearm. M, M', The biceps. As a lever: A. 

 fixed point ; 0', point of application of the resistance (the hand) ; M', point of 

 application of the force. 



30 Diagram of the 



