718 



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. m.-A horizontal section of a ° Stl ' ich » l"2,000th of an inch, 



scale, or flattened spine, from Wiethe breadth 1-6, 000th. 

 skin of a Trygon, or Sti7ig Ray ; this j + i r> . •; • .1 r 

 exhibits large Haversian canals, ln ine -aeptUlO, tile DOnes 

 with numerous wavy parallel tubes, ma y De either hollow, Cancel- 

 like those of dentine, cominunicat- ■, , , t j i ,, . n 



iug with them. This specimen lated, Or SOlld ; but the specific 



shows, besides these wavy tubes, CT ravitV is Tint «?n crrPMr n« that 

 numerous bone-cells, whose cana- gravity IS UOl SO great as tiiat 



licuii communicate with the tubes, of birds or mammals. The 



as in many specimens of dentine. short bones rf most q{ ^ 



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 



