424 MUSCULAR MOTION. 



in order .that the lever may assume the position indicated by the 

 dotted lines F w, the weight or resistance is moving through the much 

 more considerable space W w. 



The direction in which the power is inserted into the lever likewise 

 demands notice. When perpendicular to the lever, it acts with the 

 greatest advantage, the whole of the force developed being employed 

 in surmounting the resistance ; whilst if inserted obliquely a part of 

 the force is employed in tending to move the lever in its own direc- 

 tion; and this part is destroyed by the resistance of the fulcrum. 



Lastly: the general principles of equilibrium in levers consist in 

 this ; that whatever may be the direction in which the power and re- 

 sistance are acting, they must always be to one another inversely as 

 the perpendiculars drawn from the fulcrum to their lines of direction. 

 In Fig. 176, for example, the line of direction of the upper weight is 

 W w\ that of the power P p', and, to keep the lever in equilibrium in 

 this position, the forces must be to one another inversely as F w to F p. 



In applying these mechanical principles to the illustration of muscu- 

 lar motion, we must, in the first place, regard each movable bone as a 

 lever, whose fulcrum or centre of motion is in its joint ; the power at 

 the insertion of the muscle ; and the resistance in its own weight and 

 that of the parts which it supports. In different parts of the skeleton 

 we find the three kinds of levers. Each of the vertebrae of the back 

 forms, with the one immediately beneath it, a lever of the first kind, 

 the fulcrum being seated in the middle of the under surface of the 

 body of the vertebra. The foot, when we stand upon the toe, is a 

 lever of the second kind, the fulcrum being in the part of the toes 

 resting upon the soil ; the power in the muscles inserted into the heel, 

 and the resistance in the ankle-joint, on which the whole weight of the 

 body rests. Of levers of the third kind we have numerous instances ; 

 of which the deltoid, to be described presently, is one. In this, as in 

 other cases, the applicability of the principle, laid down regarding the 

 arms of the lever,, &c., is seen, and we find, that, in the generality of 

 cases, the power is inserted into the lever so near to the fulcrum, that 

 considerable force must be exerted to raise an inconsiderable weight ; 

 that so far, consequently, mechanical disadvantage results; but such 

 disadvantage enters into the economy of nature, and is attended 

 with so many valuable concomitants as to compensate richly for the 

 expense of power. Some of these causes, that tend to diminish the 

 effect of the forces, we shall first consider, and afterwards attempt to 

 show the advantages resulting from these and similar arrangements in 

 effecting the wonderful, complicate operations of the muscular system. 

 In elucidation of this subject, we may take, with Haller, 1 the case of 

 the deltoid the large muscle, which constitutes the fleshy mass on the 

 top of the arm, and whose office it is to raise the upper extremity. 

 Let W F, Fig. 177, represent the os humeri, with a weight W at the 

 elbow, to be raised by the deltoid D. The fulcrum F is necessarily, 

 in this case, in the shoulder joint; and the muscle D is inserted much 



1 Elementa Physiologiee, lib. xi. 2. 



