718 



THE MICROSCOPE. 



cellated structure will he found to reseiiiLle tlie 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 

 t^^ 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 

 arrf.nofement 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 

 lemith of a bone-cell of the 



Fig. 852.-^ norizontal section o/ a O^trich is l-2,000th of an iuch, 

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

 slcin of a Trygon, or Sting Ray; this j ,i Pfi^^f/J/n fliP hnnpc 



exhiliits large Haversian canals, J-H 1110 lieptUia, ttie DOUCS 



witli numerous wavy paralleltubes, i-jiay be either hollow, Cancel- 

 like those of dentine, communicat- , '', ,., n.ji ■ n 



ing with them. This specimen latcd, Or SOlld ; DUt the SpeClhC 



shows, besides these wavy tubes, <-n,r,vitv iq i-int <5n o-rpnf n<5 tVinf. 

 numerous bone-cells, whose cana- 8^^^^]'} ^^ ^^^t SO grCdl aS inai 



liculi communicate with the tubes, of birds Or mauimals. The 

 as in many specimens of dentine. i j. i, r* i. r A^ 



^ ^ short bones oi most ol the 



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 tlie 



