384 THE MICROSCOPE. 



office of an Haversiaii 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 

 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, we shall find the canals and the bone-cells 

 arranged both vertically and longitudinally : the bone- cells are most 

 remarkable for the great size to which they attain ; in the turtle they 

 are l-375th of an inch in length: the canaliculi, too, are extremely nu- 

 merous, 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 ; and there 

 are certain remarkable peculiarities in the bones of fishes which are so 

 characteristic, that a bone of one of these creatures can never be con- 

 founded with that of any animal of a higher class, when once the true 

 structure has been satisfactorily understood. Of all the varieties of 

 structure in the bones of fishes, by far the greater number exhibit no- 

 thing 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 ; others consist of Haversian canals, 

 with fine tubes and bone-cells ; whilst 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 canali- 

 culi, which are often of a very large size. The bone-cells are remarkable 

 for their graduate figure, and for the canaliculi which are derived from 

 their being few in number ; they are readily seen to anastomose freely 

 with the canaliculi given off from neighbouring cells; and if the speci- 

 men under examination be a thin layer of bone, such as the scale of an 

 osseous fish, from the cells lying nearly all in one plane, the anastomose 

 of the canaliculi will be rendered beautifully distinct. In the hard 

 scales of many of the osseous fishes, such as the Lepidosieus and Cal- 

 licthys, and in the spines of the /Siluridce, the bone-cells are beautifully 

 seen ; in the true bony scales comprising the exo-skeleton of the carti- 

 laginous fishes, the bone-cells are to be seen in great numbers. In the 

 spines of some of the Ray family may be noticed a peculiar structure : 

 the Haversian canals are large and very numerous, and communicating 



