170 BONE AND BONE MAEEOW 



minute canals, the canaliculi, radiate in all directions, thus plac- 

 ing the lacuna in open communication with its neighbors, and 

 eventually with the lymphatic spaces of the central Haversian canal. 

 The branching processes of the bone corpuscles frequently pro- 

 ject for a short distance into the canaliculi. These cytoplasmic 

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

 retracted and the corpuscles 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 frequently branch to permit a cor- 

 responding division of their blood vessels, and all of the Haversian 

 canals are connected either directly or indirectly with the peri- 

 osteum, the nutrient foraminae, or the marrow cavity, from the 

 blood vessels of which their vascular supply is derived. 



The interstitial lamellae are likewise composed of dense inter- 

 lacing bundles of calcined fibrous tissue, within and between which 

 are lacunae, canaliculi, and bone corpuscles, all disposed in a man- 

 ner exactly similar to their arrangement within the concentric 

 lamellae of the Haversian systems. Coursing through the inter- 

 stitial lamellae are Volkman's canals, which are similar in origin, 

 contents, and function to the Haversian canals but which are 

 not surrounded by concentric lamellae. Volkman's canals fre- 

 quently arise as branches of the Haversian canals which wander 

 out, as it were, into the interstitial lamellae. 



The circumferential lamellae do not differ in structure from the 

 other bony lamellae. They possess the same arrangement of lami- 

 nated calcareous connective tissue, with lacunae, canaliculi, and 

 bone corpuscles, as in the concentric and interstitial lamellae. 

 Even more than elsewhere, however, the circumferential lamellae 

 are firmly bound together by elastic fibres which pass from the 

 periosteum into and through the superficial lamellae; these are 

 known as the perforating fibres of Sharpey. Similar fibres connect 

 together the concentric and interstitial lamellae. The perforating 

 elastic fibres are frequently surrounded by an envelope of fibrous 

 connective tissue. 



BONE MARROW. Bone marrow consists of a variety of con- 

 nective tissue which is rich in fat cells and blood vessels and 

 which also contains osteogenic and hematopoietic elements, the 



