BONE. 



[ 108 ] 



BONE. 



bone is the hard substance serving to give 

 firmness to the bodies of the Vertebrata, to 

 protect their delicate organs, and to form 

 points of attachment for muscles. 



To the naked eye, bone appears to consist 



Fig. 64. 



Magnified 60 diameters. 



Haversian canals from the superficial layers of a 

 human femur, at eighteen years of age, treated with 

 muriatic acid, a, Haverdian canals; 6, osseous sub- 

 stance with lacunae. 



Fig. 65. 



**' 



d' 



c 



Magnified 25 diameters. 



Segment of a transverse section of the shaft of the human femur, at eigh- 

 teen years of age. a, Haversian canals; 6, their internal orifices; c, the 

 external orifices ; d, osseous substance, with lacunae. There are no trans- 

 verse sections of the Haversian canals, nor concentric laminae. 



of an apparently homogeneous basis, sur- 

 rounding certain cavities, areolae or can<rUi 

 these are most numerous and larger towards 

 the centre, where, in the Mammalia and Birds, 

 they form a larger cavity called the medullary 

 canal. This contains the marrow in the 

 former class, but air in the latter. Hence 

 we'recognize in bone an outer compact and 

 an inner spongy portion. 



On examining a thin transverse section of 

 bone under the microscope by transmitted 

 light and with a low power, it is found to 

 exhibit a number of round or oval apertures; 

 these are the orifices of the divided vascular 

 or Haversian canals (fig. 63 c). These canals 

 contain blood-vessels in the natural state. 

 They are cylindrical, sometimes flattened, 

 communicate freely with each other and the 

 medullary canal, and also open upon the 

 outer surface of the bone. They mostly run 

 parallel with the axis in the long bones ; 

 whilst in the flat bones they are parallel to 

 the surfaces, frequently following a radiating 

 course. The branches by which they com- 

 municate with each other 

 are either transverse or ob- 

 lique, and pursue a radiating 

 or tangential course. 



Hence in a longitudinal 

 or superficial section of bone, 

 the canals are seen running 

 longitudinally, here and 

 there connected by anasto- 

 mosing branches, and form- 

 ing elongated somewhat 

 rectangular meshes (fig. 64). 

 In transverse sections of 

 foetal and incompletely de- 

 veloped bones, scarcely any 

 of the apertures aremet'with, 

 but the canals are seen pur- 

 suing a tangential or radial 

 course (tig. 65 a) ; so that 

 the bones appear to consist 

 of short thick layers, each of 

 which belongs to two canals, 

 which separation is also in- 

 dicated 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 la- 

 minated structure. The 

 laminae are visible in sec- 

 tions of dried bone (fig. 63 

 , 6), but much more 



