53 



absent, and the bone-cells are very numerous, but the canaliculi pro- 

 ceeding from them are very few in number, and the granular matter is 

 very well seen ; but when the crania are thicker, there are two plates 

 of bone, each containing bone-cells, with a system of Haversian ca- 

 nals between, forming a diploe or cancellated structure. In some of 

 the small birds, I have noticed that small tubes of bone, each pro- 

 vided with the characteristic bone-cells, extend inwards towards the 

 dura mater, but the precise office which these tubes perform I cannot 

 at present determine. A striking contrast between the transverse sec- 

 tion of the long bone of a bird and that of a mammal will be exhibited 

 when they are viewed with a power of about 50 linear ; that of the 

 mammal will show the Haversian canals with the concentric laminae 

 around them, whilst that of the bird will appear to be composed of 

 large curved laminae, which follow the direction of the outer margin 

 of the bone which is due to the Haversian canals, running for the 

 most part transversely : the concentric arrangement around the Ha- 

 versian canals, like that of the Mammalia, is only to be seen when 

 the canals run at right angles to the section. 



In the Reptilia, the bones may be either hollow, cancellated, or 

 solid, and, generally speaking, whichever form prevails, the bone 

 may be said to be very compact and heavy, but the specific gravity 

 not so great as that of birds or mammals. The short bones of most 

 of the Chelonian reptiles are solid, but the long bones of the ex- 

 tremities 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 one of the 

 long bones, and contrasted with that of a mammal or bird, we shall 

 notice at once the differences which the reptile presents ; there are 

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

 one view, in the same 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 -jy 7 th of an inch in length : the canaliculi, too, are ex- 

 tremely numerous, and are of a size proportionate to that of the bone- 

 cell. In most bones, especially the flat bones, or thin plates taken 

 from any of the flat bones, the cells are arranged most frequently in 

 parallel rows, without any Haversian canals : the canaliculi are of 

 very large size, and are remarkable for their numerous bifurcations; 



