ANIMAL MECHANICS. LEVERS. 163 



external point of resistance. The bones constitute the rigid rods, and, 

 with the parts attached to them, the weight to be moved. The mus- 

 cles are the source of the power. The first kind of lever is illustrated 

 in the adjustment and movement of the skull upon the first vertebra 

 of the neck, Fig. 48; the fulcrum, 1, here being in the transverse 

 plane of the two articular surfaces of the atlas ; the weight, 2, is the 

 excess in gravity of the parts of the head and face in front of the 

 joint, over the weight of the parts behind it ; whilst the power, 3, 

 resides in the muscles at the back of the neck, extending from the 

 spine to the cranium. The movements of all the vertebrae on one an- 

 other, from above downwards ; the movements of the lowest lumbar 

 vertebra on the sacrum ; of the pelvis on the thigh bones ; of the 

 thigh on the leg ; and of the leg on the ankle, are also examples of 

 the first kind of leverage ; so also is the extension of most of the joints 

 in the limbs, as the elbow, knee, ankle-joint, and knuckles of the fin- 

 gers and toes. The second kind of lever is illustrated in the foot, 

 whilst it rests upon the ground with the heel raised, Fig. 49 ; here the 



Fig. 49. 



Fig. 49. Example of a lever of the second order, shown in the lifting of the body, by raising the heel from 

 the ground. 1, the power, in the muscles of the calf; 2. the weight of the body transmitted through the 

 leg bones to the foot; 3, the fulcrum, at the ball of the great toe, where it rests on the ground. 



fulcrum, 3, is the ground or floor beneath the ball of the great toe ; 

 the weight, 2, is that of the body transmitted through the leg ; whilst 

 the power, 1, resides in the muscles of the calf, which pull by the 

 great tendo Achillis on the heel bone. The depression of the lower 

 jaw, in opening the mouth wide, affords another illustration of this 

 kind of lever. The third kind of lever, which is much more frequently 

 used in animal mechanics, is employed in raising the lower jaw, in 

 moving the ribs, in raising the collar bone and shoulder ; and especi- 

 ally in the flexion of all the joints of the limbs. The most familiar 

 example is in the elbow joint, Fig. 50; here the fulcrum, 1, is in the 

 joint ; the weight, 3, is the fore-arm and hand, without, or with, some 

 additional object in the latter ; whilst the power is in the biceps, 2, 

 and brachialis muscles, which are inserted, respectively, into the radius 

 and ulna, in front of the centre of the motion in the joint. 



The relative advantages or disadvantages of the several levers, 

 mechanically considered, depend upon the proportionate distances, in 



