56 TEXT-BOOK OF PHYSIOLOGY. 



dance in the interior of the bones. The shaft of a long bone is hollow. 

 This central cavity, which extends from one end of the bone to the 

 other, as well as the interstices of the cancellated tissue, is filled in the 

 living state with marrow. The marrow or medulla is composed of a 

 connective-tissue framework supporting blood-vessels. In its meshes 

 are to be found characteristic bone cells or osteoblasts, the function 

 of which is supposed to be the formation of bone. In the long bones 

 the marrow is yellow, from the presence in the connective-tissue 

 corpuscle of fat globules, which arise through the transformation of 

 the cell protoplasm. In the cancellated tissue, near the extremities 

 of the long bones, this fatty transformation does not take place to the 

 same extent, and the marrow appears red. The cells of the red 

 marrow are believed to give birth indirectly to the red blood-cor- 

 puscles. 



Physical and Physiologic Properties of Connective Tissues. 

 Among the physical properties may be mentioned consistency, 

 cohesion, and elasticity. Their consistency varies from the semi- 

 liquid to the solid state, and depends on the quantity of water which 

 enters into their composition. Their cohesion, except in the softer 

 varieties, is very considerable, and offers great resistance to traction, 

 pressure, torsion, etc. In all the movements of the body, in the con- 

 traction of muscles, in the performance of work, the consistence and 

 cohesion of these tissues play most important roles. Wherever the 

 various forms of connective tissue are found, their chemic com- 

 position and structure are in relation to their functions. If traction 

 be the preponderating force, the structure becomes fibrous, as in 

 ligaments and tendons, and the cohesion greatest in the longitudinal 

 direction. If pressure be exerted in all directions, as upon mem- 

 branes, the fibers interlace and 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. Ex- 

 tensibility is not a characteristic feature, except in those forms con- 

 taining 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 opposing temporarily acting forces. 



The Skeleton. The connective tissues in their entirety con- 

 stitute 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 throughout the body, connecting the various 



