BONE STRUCTURE. 719 



bone, like that of some of the birds, is remarkable for its 

 density and its whiteness. When a transverse section is 

 taken from one of the long bones, and contrasted with 

 that of a mammal or bird, we shall notice at once the 

 difference which the reptile presents : there are very few, 

 if any, Haversian canals, and these of large size ; and at 

 one view, in the section, fig. 347, we shall find the canals 

 and the bone-cells arranged both vertically and longi- 

 tudinally : the bone-cells are most remarkable for the 

 great size to which they attain ; in the Turtle they are 

 1-3 7 5th of an inch in length, the canaliculi are extremely 

 numerous, and are of a size proportionate to that of the 

 bone-cell. 



In fishes we have a greater variation in^the minute 

 structure of the skeleton than in either of the three classes 

 already noticed. Of all the varieties of structure in the 

 bones of fishes, by far the greater number exhibit nothing 

 more than a series of ramifying tubes, like those of teeth ; 

 others exhibit Haversian canals, with numerous fine tubes 

 or canaliculi, like ivory tubes, connected with them ; a few 

 consist of Haversian canals, with fine tubes and bone- 

 cells, fig. 349; and a rare form, found only as yet in the 

 sword of the Swordfish (Istiophorus), exhibits Haversian 

 canals and a concentric laminated arrangement of the 

 bone, but no bone-cells. The Haversian canals, when 

 they are present, are of large size, and very numerous, and 

 then the bone-cells are, generally speaking, either absent 

 or but few in number; their place being occupied by 

 tubes or canaliculi, which are often of a very large size. 

 The bone-cells are remarkable for their graduate figure, 

 and the canaliculi which are derived from them are few in 

 number; they are seen to anastomose freely with the 

 canaliculi given off from neighbouring cells; and if the 

 specimen under examination is a thin layer of bone, such 

 as the scale of an osseous fish, from the cells lying nearly 

 all in one plane, the anastomoses of the canaliculi are 

 seen beautifully distinct. In the hard scales of many of 

 the osseous fishes, such as the Lepidosteus and Calicthys, 

 and in the spines of the Siluridce, the bone-cells are 

 beautifully seen ; in the true bony scales comprising the 

 exo-skeleton of the cartilaginous fishes, the bone-cells are 



