HISTOLOGY OF EPITHELIAL AND CONNECTIVE TISSUES. 37 



through which lymph circulates destined for the nourishment of bone. 

 Each lacuna contains the bone corpuscle, which bears a close resemblance to 

 the usual branched connective-tissue corpuscle, and whose function appears 

 to be the maintenance of the nutrition of the bone. 



The surface of every bone in the recent state is invested with a fibrous 

 membrane, the periosteum, except where it is covered with cartilage. The 

 inner surface of this membrane is loose in texture, and supports a fine plexus 

 of capillary blood-vessels and numerous protoplasmic cells the osteoblasts. 

 As this layer is directly concerned in the formation of bone, it is spoken of as 

 the osteogenetic layer. 



A section of a bone shows that it is composed of two kinds of tissue com- 

 pact and cancellated. The compact is dense, resembling ivory, and is found 

 on the outer portion of the bone; the cancellated is spongy, and appears to be 

 made up of thin, bony plates, which intersect one another in all directions, 

 and is found in greatest abundance 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 lobules, 

 which arise through the transformation of the cell protoplasm. In the can- 

 cellated 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- 

 corpuscles. 



Physical and Physiologic Properties of Connective Tissues. 



Among the physical properties may be mentioned consistency, cohesion, 

 and elasticity. Their consistency varies from the semiliquid to the solid 

 state, and depends on the quantity of water which enters into their compo- 

 position. Their cohesion, except in the softer varieties, is very considerable, 

 and offers great resistence to traction, pressure, torsion, etc. In all the move- 

 ments of the body, in the contraction 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 

 composition 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 membranes, the fibers interlace and 



