PHYSIOLOGY OF THE SKELETON. 43 



THE PHYSIOLOGY OF THE SKELETON. 



The animal body is characterized by the power of executing a great 

 variety of movements, all of which have reference to a change of 

 relation of one part of the body to another, or to a change of position 

 of the individual in space, as in the various acts of locomotion. If 

 in the execution of these movements the different parts are applied 

 or directed to the overcoming of oppqsing forces in the environment, 

 the animal is said to be doing work. In the conception of the 

 animal body as a machine for the accomplishment of work the 

 skeleton, the muscle and nerve tissues constitute the three primary 

 mechanisms, all of which bear certain definite relations one to 

 another. 



The Skeleton is the passive framework, the axial portion of which 

 (the vertebral column, head, ribs, and sternum) impart more or less 

 fixity and rigidity, while the appendicular portions (the bones of the 

 arms and legs) impart extreme mobility. The bones of the arms 

 and legs more especially may be looked upon as constituting a sys- 

 tem of levers, the fulcra of which, the points of rest around which 

 they move, lie in the joints. 



That a lever may be effective as an instrument for the accomplish- 

 ment of work, it must not only be capable of moving around its ful- 

 crum, but it must at the same time be acted on by two opposing 

 forces, one passive, the other active. In the movement of the bony 

 levers of the animal body, the passive forces are largely those con- 

 nected with the environment, e. g., gravity, cohesion, friction, elas- 

 ticity, etc. The active forces by which these latter are opposed and 

 overcome through the intermediation of the bony levers are found in 

 the muscles attached to them. For the execution of all these move- 

 ments, it is essential that the relation of the various portions of the 

 bony skeleton to one another shall be such as to permit of movement 

 while yet retaining close apposition. This is accomplished by the 

 mechanical conditions which have been evolved at the points of union 

 of bones, and which are technically known as articulations or joints. 



A consideration of the body movements involves an account of (i) 

 the static conditions, or those states of equilibrium in which the body 

 is at rest e. g., standing, sitting; (2) the dynamic conditions, or 

 those states of activity characterized by movement e. g., walking, 

 running, etc. In this connection, however, only those physical and - 



