168 THE OSSEOUS TISSUE. 



medullary cells. The cellular structure is only found to 

 exist when ossification commences. The relative propor- 

 tion of these two substances varies greatly even in the same 

 bone ; the compact predominating, as in the body of long 

 bones, where strength is required, while the spongy pre- 

 vails at the extremities, where extent of surface, for va- 

 riety of motion, is needed. 



The microscope makes more evident the laminated con- 

 dition of bone, and reveals many other points in minute 

 anatomy hitherto unknown. It shows the laminated struc- 

 ture in long bones to FIG. 31. 

 be arranged in concen- 

 tric circles, while in the 

 flat, the fibres run par- 

 allel with the surface. 

 Between the laminae a 

 multitude of longitu- 

 dinal canals, called, af- 

 ter Havers, their dis- 

 coverer, the Haversian canals, are seen. They are nar- 

 row, cylindrical, form a net-work, connect with e-ach other, 

 and open externally upon the periosteum, and internally 

 connect with the cellular structure and medullary cells. 

 These canals are fo-und to measure from 1-200 to 1-2500 of 

 an inch, and contain blood-vessels and an oily matter, and 

 are supposed to be simply miniature representations of the 

 great medullary canal itself, having similar functions of 

 receiving blood vessels and containing the fat. 



These Haversian or medullary canals are seen to be 

 surrounded by from four to twelve concentric lamella or 

 osseous plates, intersected or perforated by minute tubes 

 called calcigerouSy which are supposed to contain the earthy 

 matter; also between the laminae are seen, on a trans- 

 Terse section of bone, little cavities called, after their 

 discoverer, the corpuscles of Purkinje, which also receive 

 the names of bone corpuscles and lacunas. (Fig. 32.) These- 



FIG. 31 represents the concentric lamellae of bone, taken from a transverse 

 section of the tibia, after maceration in weak muriatic acid. 



