Histology 325 



nective tissue. Elastic cartilage contains numerous elastic fibers. 

 Calcified cartilage is rendered white and relatively hard by deposit 

 of calcium salts in the matrix. 



Bone. Cartilage and bone arc similar in dial their essential skeletal 

 material is a nonliving matrix within which are embedded living cells. 

 Bone differs from cartilage in that the matrix is highly calcified and 

 correspondingly hard, and also in that it never exhibits the apparent 

 homogeneity of the matrix of hyaline cartilage but is disposed in very 

 thin parallel layers. Usually the deeper substance of a bone is of a 

 porous or spongy texture (cancellous bone), while the outer region 

 is dense or solid (compact bone). 



A section of fully developed compact bone, seen under high mag- 

 nification, shows the matrix layers or lamellas arranged in parallel or 

 concentric order (Fig. 267 B, 268). Flattened between adjacent lamellas 

 are minute cavities, the lacunas. Exceedingly fine canals, the cana- 

 liculi, extend between each lacuna and neighboring lacunas, piercing 

 the intervening lamellas. In bone of a living animal, each lacuna is 

 occupied by a living bone-cell (osteoblast) from which processes ex- 

 tend into the adjoining canaliculi. Therefore bone is richly occupied 

 by living material. 



All external surfaces of bone are covered by a membrane, the peri- 

 osteum, of dense fibrous connective tissue well supplied with blood- 

 vessels which enter the bone and branch throughout it. Most bones, 

 notably the long bones of the appendages, have internal cavities occu- 

 pied by a more or less vascular soft tissue, the marrow. The "yellow 

 marrow " of long bones contains much fat. "Red marrow" is highly 

 vascular, contains little fat, and may be a source of blood-cells of 

 various types. 



Blood-vessels from both the periosteum and the marrow enter and 

 branch throughout the bone. From these vessels, substances necessary 

 for the metabolism of the bone-cells diffuse through the system of 

 connected lacunar spaces. 



In long bones the larger blood-vessels lie approximately parallel to 

 the long axis of the bone. Around such vessels the bone lamellas are 

 arranged in concentric order (Figs. 267, 268) forming so-called Haver- 

 sian systems. These concentric systems are much less prominently 

 developed in flat bones. 



The matrix of bone consists of commingled organic and inorganic 

 materials. Collagenous and other protein substances constitute the 

 organic part while various salts of calcium, mostly the phosphate and 

 carbonate, are the most important inorganic ingredients. Bone, be- 

 cause of the rigidity of its calcified matrix, is incapable of such inter- 

 stitial growth as occurs in cartilage. A further difference between 



