76 



CONNECTIVE TISSUE CAETILAGE BONE 



radiate in all directions, thus placing the lacuna in open communication 

 with its neighbors, and eventually with the lymph spaces of the cen- 

 tral Haversian canal. The branching processes of the bone cells fre- 

 quently project for a short distance into the canaliculi. These cyto- 

 plasmic branches are more numerous in newly formed bone, later they 

 are retracted and the cells become more or less shriveled in appearance. 

 The Haversian system, being developed about a central canal which 

 marks the course of a blood-vessel, necessarily acquires a slender columnar 

 shape, its long axis being usually disposed in a direction nearly parallel 

 to that of the bone of which it forms a part. The Haversian canals fre- 

 quently branch to permit a corre- 

 sponding division of their blood-ves- 

 sels, and all of the Haversian canals 

 are connected either directly or indi- 

 rectly with the periosteum, the nu- 

 trient foramina, or the marrow cavity 

 thus forming a complete connected 

 system between marrow cavity and 

 surface from the blood-vessels of 

 which -their vascular supply is de- 

 rived. 



INTERSTITIAL LAMELLAE. The in- 

 terstitial lamellae are likewise com- 

 posed of dense interlacing bundles of 

 calcined fibrous tissue, within and be- 

 tween which are lacunae, canaliculi, 

 and bone cells, all disposed in a man- 

 ner exactly similar to their arrangement within the concentric lamellse 

 of the Haversian systems. Coursing through the interstitial and circum- 

 ferential lamellae are Vollcmanns canals, which are similar in origin, con- 

 tents, and function to the Haversian canals but which are not surrounded 

 by concentric lamellae. Volkmann's canals frequently arise as branches of 

 the Haversian canals which wander out, as it were, into the interstitial 

 lamellae. 



CIRCUMFERENTIAL LAMELLA;. The circumferential lamellae do not 

 differ in structure from the other osseous lamellae. They possess the 

 same arrangement of laminated calcareous connective tissue, with lacunae, 

 canaliculi, and bone cells, as in the concentric and interstitial lamellae. 

 Even more than elsewhere, however, the outer circumferential lamellae are 

 firmly bound together by collagenous and elastic fibers which pass from 



FIG. 90. TRANSVERSE SECTION OP 

 HAVERSIAN CANAL, WITH CONTENTS. 



a, arteriole; v, venule; I, lymphatic; 

 n, non-medullated nerve fibers; c, 

 bone cell. (After Schafer.) 



