ris 



THE MICROSCOPE. 



collated structure will 'be found to resemble the cancellated 

 structure in the long bones viz. thin plates of bone, with 

 one layer of bone-cells without Haversian canals ; the outer 

 layer will exhibit Haversian canals of large size, with bone- 

 cells of large size, and a slightly laminated arrangement , 

 but the inner or vitreous layer resembles the densest bone, 

 as the outer part of the shaft of a long bone, for instance, 

 and will exhibit both smaller Haversian canals and more 

 numerous bone-cells of ordinary shape around them. 



A transverse section of the long bone of a bird, when 

 contrasted with that of a mammal, exhibits the following 

 peculiarities : the Haversian 

 canals are more abundant, 

 much smaller, and often run 

 in a direction at right angles 

 to that of the shaft, by which 

 means the concentric laminated 

 arrangement is in some cases 

 lost ; the direction of the canals 

 follows the curve of the bone ; 

 the bone-cells also are much 

 smaller and more numerous ; 

 but the number of canaliculi 

 given off from each of the cells 

 is less than from those of mam- 

 mals, fig. 348 : the average 

 length of a bone-cell of the 



Fig. 352.-^ Horizontal section o/a^trich is l-2,000thof an inch, 

 scale, or flattened spine, from the the breadth l-6,000tll. 



fxSbS S^lSn^S I* *"> ZepM, the bones 



with numerous wavy parallel tubes, ma y b e either hollow. Cancel- 

 like those of dentine, communicat- , . 7 1 ,., . ,,-,' 



ing with them. This specimen iated, Or solid ; but the specific 

 shows, besides these wavy tubes, o-ravitv is nor <?n rrrpif aa fhaf 

 numerous bone-cells, whose cana- S ravi [y 1S nol} so S reat as tnat 

 liculi communicate with the tubes, of birds or mammals. TllO 

 deutine. 



Chelonian reptiles are solid, but the long bones of the 

 extremities are either hollow or cancellated; the ribs of 

 the Serpent tribe are hollow, the medullary cavity per- 

 forming the office of an Haversian canal ; the bone-cells 

 are accordingly arranged in concentric circles around the 

 canal The vertebrae of these animals are solid ; and the 



