CHAPTER VI. 

 THE PHYSIOLOGY OF THE SKELETON. 



The skeleton in its entirety determines the plan of organization of the 

 animal body. Its axial portion is the foundation element and the center 

 around which the appendicular portions are developed and arranged with 

 a certain degree of conformity. The character and the arrangement of the 

 bones of the axial portion endow the animal mechanism with a certain degree 

 of fixity, combined with slight mobility, while the character and arrangement 

 of the bones of the appendicular portions endow it with extreme mobility. 

 The bones collectively constitute a system of levers, the fulcra of which lie 

 in the points of union of the bones, and with which the animal is enabled to 

 execute a variety of movements, to change its position relatively to its environ- 

 ment and overcome opposing forces. The structure and the chemic com- 

 position of the bones, consisting as they do of inorganic matter 67 per cent, 

 and of organic matter 33 per cent, endow them with both rigidity and elastic- 

 ity, physical properties which admirably adapt them to the character of the 

 work necessitated by the environment and the organization of the animal. 

 The rigidity of bone is considerable as compared with other hard and rigid 

 materials. The breaking limit, in terms of the weight in kilos required to 

 tear across a rod one square millimeter in cross-section of various materials is 

 as follows: Cast iron 13; bone 12; oak 6.5; granite 1.9. The elasticity is 

 about one-sixth that of wrought iron and twice that of oak parallel to the 

 grain (MacAlister) . In youth bones are quite elastic; in old age they are 

 fragile because of a diminution of osseous tissue and an increased porosity 

 and, therefore, at both periods less capable of functionating as effectively 

 as in the middle period of life. The skeleton also serves for the attach- 

 ment of muscles and affords support and protection to viscera. 



For the manifestation of the activities of the animal 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 



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