BONE. 



[ 89 ] 



BONE. 



recognize in bone an outer 

 compact and an inner spongy 

 portion. 



On examining a thin trans- 

 verse section of bone under 

 the microscope by transmitted 

 light and with a low power, 

 it is found to exhibit a num- 

 ber of round or oval apertures; 

 these are the orifices of the 

 divided vascular or Haversian 

 canals (fig. 64 c). These ca- 

 nals contain blood-vessels in 

 the natural state. They are 

 cylindrical, sometimes flat- 

 tened, communicate freely 

 with each other and the me- 

 dullary canal, and also open 

 upon the outer surface of the 

 bone. . They mostly run par- 

 allel with the axis in the long 

 bones ; whilst in the flat bones 

 they are parallel to the sur- 

 faces, frequently following a 

 radiating course. Thebranches 

 by which they communicate 

 with each other are either 

 transverse or oblique, and 

 pursue a radiating or tangen- 

 tial course. 



Hence in a longitudinal or 

 superficial section of bone, 

 the canals are seen running 

 longitudinally, here and there 

 connected by anastomosing branches, and 

 forming elongated somewhat rectangular 

 meshes (fig. 66.). 



In transverse sections of foetal and incom- 

 pletely developed bones, scarcely any of the 



Fig. 65. 



Magnified 60 diameters. 



Haversian canals from the superficial layers of a human 

 femur, at eighteen years of aj-e, treated with muriatic acid. 

 ti, Haversian canals ; b, osseous substance with lacunae. 



Magnified 25 diameters. 



Segment of a transverse section of the shaft of the human femur, at eighteen 

 years of age. , Haversian canals ; b, their internal orifices ; c, the external 

 orifices ; d, osseous substance, with lacunae. There are no transverse sections 

 of the Haversian canals, nor concentric laminae. 



apertures are met with, but the canals are 

 seen pursuing a tangential or radial course 

 (fig. 65 a) ; so that the bones appear to con- 

 sist of short thick layers, each of which be- 

 longs to two canals, which separation is also 

 indicated by a faint median line in each 

 layer. 



The Haversian canals vary considerably in 

 size, from about 1-1000 to 1-200". 



The osseous substance or basis of bone 

 possesses a laminated structure. The lami- 

 nae are visible in sections of dried bone (fig. 

 64 a, &), but much more distinctly in bone 

 from which the inorganic matter has been 

 removed by digestion in dilute muriatic acid. 

 In this, the lamina? are easily separable. 

 They frequently exhibit a fibrous appear- 

 ance, and near the surfaces of the bones they 

 run parallel with these surfaces (fig. 64 6), 

 but in the other portions they mostly sur- 

 round the Haversian canals concentrically 

 (fig. 64 e). 



When a section of bone is examined with 

 a somewhat high power, it exhibits nu- 



