656 ANIMAL PHYSIOLOGY. 
powerful lever. This is illustrated by the action of the biceps 
on the forearm. 
It is to be remembered that, while the flexors and extensors 
of a limb act in a certain degree the opposite of one another, 
Beg ‘ 
hy | 
Dy) 
Fig. 491.—Skeleton of deer. The bones in the extremities of this the fleetest of quadrupeds 
are inclined very obliquely toward each other and toward the scapular and iliac bones. 
This arrangement increases the leverage of the muscular system and confers great 
rapidity on the moving parts. It augments elasticity, diminishes shock, and indirectly 
begets continuity of movement. a, angle formed by femur with ilium ; b, angle formed 
by tibia and fibula with femur ; c, angle formed by phalanges with cannon-bone; e, angle 
formed by humerus with scapula; f, angle formed by radius and ulna with humerus 
(Pettigrew). 
there is also, in all cases perhaps, a united action; the one 
set, however, preponderating over the other, and usually sev- 
eral muscles, whether of the same or different classes, act 
together. 
Standing itself requires the exercise of a large number of 
similar and antagonistic muscles so co-ordinated that the line 
of gravity falls within the area of the feet. An unconscious 
person falls, which is itself an evidence of the truth of the 
above remarks. 
The following statements in regard to the direction of the 
line of gravity may prove useful: 1. That for the head falls in 
front of the occipital articulation, as exemplified by the nod- 
ding of the head in a drowsy person occupying the sitting atti- 
tude. 2. That for the head and trunk together passes behind a 
line joining the centers of the two hip-joints, hence the uncor- 
rected tendency of the erect body of man is to fall backward. 
3. That for the head, trunk, and thighs falls behind the knee- 
