chap, vii.) BONE. 53 



in open and free communication with the lymphatic 

 vessels of the marrow spaces and of the Haversian 

 canals. 



65. In the bone matrix, each lacuna contains also a 

 nucleated protoplasmic cell, called the bone cell, which, 

 however, does not fill it completely. In the young state, 

 the cell is branched, the branches passing into the 

 canaliculi of the lacunae ; but in the old state very few 

 processes can be detected on a bone cell, which, with 

 its lacuna and canaliculi, is called a bone corpuscle. 



66. According to the arrangement of the bone 

 substance, we distinguish compact from spongy sub- 

 stance. The former occurs in the shaft of tubular 

 bones and in the outer layer of flat and short bones. 

 Its lamellae are arranged as : (<z) concentric or Haversian 

 lamella?, directly surrounding the Haversian canals 

 (Fig. 33 A). These are narrow canals of varying 

 lengths pervading the compact substance in a longitu- 

 dinal direction, and anastomosing with one another by 

 transverse or oblique branches. The Haversian canals 

 near the marrow cavity are larger than those near 

 the periosteum. In fact, those next to the marrow 

 cavity become gradually enlarged by absorption, until 

 finally they are fused with the marrow cavity. Each 

 Haversian canal contains a blood-vessel, one or two 

 lymphatics, and a variable amount of marrow tissue. 

 These canals open both into the marrow cavity and on 

 the outer surface into the osteogenetic layer of the peri- 

 osteum, and they form the means by which the latter 

 remains in continuity with the marrow. Each canal 

 is surrounded by a series of concentric bone lamella?, 

 with the bone corpuscles between them, and this is a 

 system of concentric lamellce. Near the external sur- 

 face of the compact substance the number of lamellae 

 in each system is smaller than in the deeper parts. 

 (6) Between these systems of concentric lamellae are 

 the intermediate or ground lamellce; they run in various 



