IN THE LIMBS OF MAMMALIA. 



69 



Two wrongs do not make a right, and when we are in doubt, it is better to follow Nature 

 strictly and try to discover her way of reconciling apparent discrepancies, than to assume 

 what is not the case in order to conform to a preestablished theory. 



The mechanical conditions necessary for making use of muscular contractility are two 

 points, one generally fixed and the other movable, to which the extremities of the muscle 

 are attached, as in the face, where the muscles may arise from the bone and be inserted 

 directly into the skin, or other part to be moved. In this way, of course, all the power of 

 the muscle is utilized ; but for the various requirements as to rapidity of action and beauty 

 of proportion, a part of the power is more often sacrificed by the introduction of two other 

 points, — one fixed, the fulcrum, and one movable, like the part to be moved, but nearer 

 to or farther from the fulcrum, and into which now the muscle is inserted. 



This is the more usual method of applying muscular contraction, and converts the vari- 

 ous segments of a limb into levers, which are so far strictly mechanical. Thus, in flexion 

 of the fore-arm, the biceps, and brachialis anticus do not reach directly from their origins on 

 the scapula and humerus to the hand, which is the part to be moved, but two other points 

 are introduced, — one fixed as a fulcrum, the elbow-joint, the other movable at the insertion 

 of the muscles on the fore-arm, thus between the fulcrum and the part to be moved, which 

 represents the weight. 



Levers are of three kinds. The first is where the fulcrum lies between the power and 

 the weight ; of this kind are all segments of the limbs when acted upon by extensor mus- 

 cles. The third kind is where the power is applied between the fulcrum and the weight 

 and to this class belong the same segments when acted upon by flexor muscles. The 

 second kind of lever is where the weight lies between the power and the fulcrum ■ of this 

 there are no examples in the body acting by and upon itself, but whenever extension of 

 a limb is employed for raising or supporting the body from the earth, then the different 

 segments from levers of the first become levers of the second kind. Thus, in rising on 

 tiptoe, the weight of the body rests upon the ankle, and so between the ball of the 

 foot, which, resting on the earth, forms the fulcrum, and the heel, to which by the 

 tendo Achillis is applied the power of the gastrocnemius and solcus muscles. I subjoin line 

 illustrations of these three kinds of levers. 



Adduction and abduction are only lateral flexion and extension; circumduction is a com- 

 pound movement resulting from gradual and successive direct and lateral flexion and 

 extension ; even rotation is essentially a peculiar form of the same two movements, for the 

 power is applied at the periphery of the rotated bone or limb, and so at one end of an imag- 

 inary diameter line through the centre, which latter represents the fulcrum ; the weight 

 may be regarded either as suspended from the half of the line or lever furthest from the 

 power, in which case the diameter line would be a lever of the first kind", or as sustained 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I. 



18 



