BONE. 



579 



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 

 follow 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. 318 : the average 

 length of a bone-cell of the 



Ostrich is 1 -2000th Of an inch, Fig. 322. A horizontal section of -a 



the breadth l-6000th. 



" In the Reptilia, the bones 

 may be either hollow, cancel- 

 lated, or solid; and, generally in s wit j *J em - This specimen 



' i . , n J shows, besides these wavy tubes, 



Speaking, Whichever form pre- numerous bone-cells, whose cana- 



Vaih thp hftnp mav hp said tr 

 vailS, tne Done may De Sam tO 



be very compact and heavy, 

 but the specific gravity is not so great as that of birds 

 or mammals. The short bones of most of the Chelouian 

 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 performing 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 bone, like 

 that of some of the birds, is remarkable for its density and 

 its whiteness. When a transverse section is taken from 

 p p 2 



scale^ or flattened spine, from the 

 skin of a Trygon, or Sting Ray ; this 

 exhibits large Haversian canals, 

 with numerous wavy parallel tubes, 

 like those of dentine, communicat- 



Hculi communicate with the tubes, 

 a s in many specimens of dentine 



