CHAPTER V. 

 THE PHYSIOLOGY OF THE SKELETON. 



The Skeleton is the passive framework of the body, the axial 

 portion of which (the vertebral column, head, ribs, and sternum) 

 imparts more or less fixity and rigidity, while the appendicular por- 

 tions (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 system 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 accom- 

 plishment of work, it must not only be capable of moving around its 

 fulcrum, 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 

 physiologic peculiarities of the skeleton, especially in its relation to 

 joints, will be referred to, which underlie and determine both the 

 static and dynamic states of the body. 



Structure of Joints. The structures entering into the formation 

 of joints are : 



i. Bones, the articulating surfaces of which are often more or less 

 expanded, especially in the case of long bones, and at the same 

 time variously modified and adapted to one another in accordance 

 with the character and extent of the movements which there 

 take place. 



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