72 STUDENTS HISTOLOGY 



of the lymph -spaces are prolonged into the eanaliculi, and 1 

 thus placed in connection with the elements of the surrounding 1 

 lacunae. 



Each lacuna contains a bone-corpuscle, the protoplasmic body 

 of which sends prolongations into the contiguous eanaliculi. In 

 the adult bone the cell is shrunken, and the processes just .men- 

 tioned are not readily demonstrable. 



The eanaliculi of any Haversian system communicate with one 

 another, but not with those of different systems. 



The concentrically placed lamellae around the Haversian canals 

 are. called Haversian lamellce. The angular area formed where 

 several Haversian systems join is filled out by interstitial lamella., 



FIG. 49. DIAGRAM OF A BONE LACUNA. 



A, A. Ground-substance of the bone. 



B, B. Limiting membrane of the bone-corpuscle within the lacuna. 



C, Nucleus and nucleolus of the corpuscle. 



D, D. Projections of the cell-body into the eanaliculi. 



while those lamellae mentioned above as occurring just below the 

 periosteum are the circumferential or fundamental lamellce. 



The arrangement of Haversian canals and their concentric- 

 lamellae is confined to compact bone. Compact bone is formed on 

 the surface of all bones, and makes up the bulk of the shaft in long 

 bones. The other variety of bone, called cancellous or spongy, 

 occurs extensively at the ends of long bones, and in short and flat 

 bones. It consists of lamellae containing lacunae and eanaliculi, 

 lying between good sized cavities, which are occupied by blood- 

 vessels and marrow, and which correspond to Haversian canals. 

 Bone is to be regarded as dense connective tissue, arranged in 

 lamellae, of which the ground-substance is impregnated with salts 

 of calcium, chiefly the phosphate, to which its hardness is due. 



At the same time, bone retains the elasticity and strength of 



