38 HUMAN PHYSIOLOGY. 



offer a uniform resistance. When pressure is exerted in a definite direction, 

 as on the extremities of the long bones, the tissue becomes expanded and 

 cancellated. The lamellae of the cancellated tissue arrange themselves in 

 curves which correspond to the direction of the greatest pressure or traction. 

 Extensibility is not a characteristic feature, except in those forms containing 

 an abundance of yellow elastic fibers. The elasticity is an essential factor 

 in many physiologic actions. It not only opposes and limits forces of 

 traction, pressure, torsion, etc., but on their cessation returns the tissues or 

 organs to their original condition. Elasticity thus assists in maintaining 

 the natural form and position of the organs by counterbalancing and oppos- 

 ing temporarily acting forces. 



The Skeleton. The connective tissues in their entirety constitute a 

 framework which presents itself under two aspects: (i) As a solid, bony 

 skeleton, situated in the trunk and limbs, affording attachment for muscles 

 and viscera; (2) as a fine, fibrous skeleton, found everywhere thoughout 

 the body, connecting the various viscera and affording support for the epi- 

 thelial, muscle, and nerve tissues. 



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 move- 

 ments the different parts are applied or directed to the overcoming of oppos- 

 ing 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 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 accomplishment 

 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 connected with the evironment, e. g., 



