ANIMAL MECHANICS. RESISTING MEDIA. 165 



is, necessarily, always an advantage of length on the side of the power, 

 subject to an equivalent loss in velocity ; and it is remarkable that 

 almost the only instance in which this lever is employed in the body, 

 is where great force is needed, viz., in lifting the body by raising the 

 heel from the ground ; for then the whole weight of the body has to 

 be lifted on one limb, as in the alternate steps in the act of walking. 

 In the third kind of lever the power necessarily acts always at a dis- 

 advantage, accompanied, however, by an invariable gain in velocity 

 on the part of the weight; and hence a moderate amount of contrac- 

 tion in the biceps of the arm, for example, see Fig. 50, moves the 

 hand at the end of the fore-arm, through a relatively large extent of 

 space. The gain in velocity, numerically estimated, is exactly pro- 

 portionate to the length of the weight-arm of the lever, as compared 

 with that of the power-arm ; in other words, to the distance between 

 the fulcrum and the weight, as compared with the distance between the 

 fulcrum and the power. If, for example, the latter distance be one 

 inch, and the former ten, then a contraction of the muscle to the ex- 

 tent of one inch will move the former through ten inches of space. 



A force acting perpendicularly on the arm of a lever, operates the 

 most advantageously ; hence, in the case of muscles acting obliquely, 

 a loss of power necessarily occurs, so that they require to be propor- 

 tionally increased in the number of their fibres, which gives them 

 greater power. Speaking generally, the flexors act more favorably 

 than the extensors, and most advantageously, as they approach the 

 utmost limits of their contraction. In the extensors, moreover, the 

 power at all times acts obliquely ; hence they are larger, or more 

 bulky, than the flexors, in the ratio of eleven to five. 



In comparing the mechanism of the various movements of the body, 

 with that of ordinary levers, it is important to note the fact, generally 

 overlooked, that, in the living body, the power exerted is intrinsic, 

 instead of being extrinsic, or applied from without, as in the common 

 levers. The animal mechanism presents, indeed, examples of the ex- 

 ercise of what are called, in mechanics, intrinsic forces. The applica- 

 tion of such a force to a lever, so as to produce motion in that lever, 

 necessitates a hinge-like action at the fulcrum, and sometimes even at 

 the seat of the weight, otherwise no motion could take place. More- 

 over, the production of this internal motion involves a loss of actual 

 lifting force, acting on the weight ; lastly, there is not, as in the use 

 of extrinsic forces, any part of the weight supported from without. 

 For these reasons, a much larger amount of muscular power has to be 

 provided for, in the intrinsic exercise of force within the body, than if 

 the force employed were extrinsic, or were applied from without. 



External Resisting Media. Forms of Progression. 



The locomotive acts of man and animals are influenced by the 

 media upon, or in, which they move, and their whole organization is 

 modified and adapted accordingly. Thus, we observe progression 

 upon the surfaces of solid bodies, performed either in air or in water ; 



