BODILY MOTION. /I 



dependent on approximation of the ends of the muscles 

 concerned. The direction and extent of these motions are 

 regulated by the forms of the movable bones, and of the 

 symphyses or of the joints by which they are connected 

 with the rest of the skeleton. The term symphysis is appli- 

 cable to the connection of two bones by a perfectly elastic 

 material, in such a way that, after having been bent or 

 twisted on each other, they tend to recover their relative 

 normal position. The only example of this in the human 

 skeleton is that of the bodies of the vertebrae. The essen- 

 tial difference between the joint and the symphysis consists 

 in this that in the former the bones have no normal 

 relation to each other, but assume with equal readiness 

 any among the infinite number of relative positions which 

 the structure of the joint allows. 



Joints are divisible into those which have a single axis 

 of rotation (hinge joints) and those which have several 

 axes (ball-and-socket joints). It is essential to the efficient 

 working of a joint of either kind (i) that the two surfaces 

 should be kept in apposition ; and (2) that the movements 

 of the bones on each other should be restrained within 

 due limits ; accordingly contrivances exist in all joints for 

 these two purposes. 



The efficiency of the action of a muscle in producing 

 motion about a joint depends on the mode of its attach- 

 ment to the bones. In all cases the effect produced is to 

 the force exerted, as the distance of the nearest point 

 of the straight line which connects the origin with the 

 insertion of the contracting muscle from the axis of 

 rotation of the joint is to the distance from the joint to the 

 insertion of the muscle. 



The maintenance of the erect posture is dependent on 

 constant muscular exertion, for the line of gravity of the 

 head falls far in front of the condyles of the occipital bone, 

 that of the head and trunk together behind the line which 

 joins the hip joints, that of the whole body, in front of the 



