52 



nonnciug an opinion upon them ; but luckily for the microscopist, the 

 denser parts of fossil bones are the last to be destroyed, and every 

 fragment he is likely to meet with will almost be certain to contain a 

 spot where the cells present their normal characters. 



In birds the majority of the bones are hollow, and the solid walls 

 thin, but very compact in structure : the medullary cavity of the shaft 

 of the long bones, which in Mammalia is either filled with marrow or 

 with a cancellated tissue, is, in birds, often smooth, and is lined by a 

 membrane, which is a continuation of that membrane which forms the 

 air-cells of the lungs, and which gains admittance into the interior of 

 the bones by a large opening, termed the pneumatic foramen, which 

 is situated in the proximal extremity of each long bone : the extremi- 

 ties of the bones are not so thickly cancellated as those of the Mam- 

 malia, but the cancelli are supplied by small tubes of bone, which 

 shoot across from one side of the wall to that of the other, like so 

 many little pillars ; everything being wisely arranged with an evident 

 view to as much lightness as is compatible with strength. In those 

 birds whose wings are not destined for flight, the pneumatic foramen 

 still exists in some of the long bones, but is absent in others. In all 

 these birds the solid walls of the bones are much thicker, and the me- 

 dullary cavity is more cancellated : to such an extent does this thick- 

 ening of the walls take place, that in the Penguin nearly every bone 

 in the body is solid, and in the Apteryx not a bone is permeated by 

 air. The bony matter, although sparingly used, is of greater specific 

 gravity in birds than in any of the other classes of animals, in conse- 

 quence of there being a larger amount of earthy matter in it ; and in 

 some bones, where the foramen exists, it almost rivals, in density and 

 in whiteness, the purest ivory ; whilst in others, where the foramen is 

 absent, the bony matter is of a yellow colour, from the oily matter 

 contained in the medullary cavity. 



A transverse section of the long bone of a bird, when contrasted 

 with that of a mammal, will exhibit the following peculiarities. The 

 Haversian canals are much more abundant and much smaller, and 

 they 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 often follows the curve of the bone ; 

 the bone-cells also are much smaller and much more numerous, but 

 the number of canaliculi given off from each of the cells is much less 

 than from those of mammals: the average length of a bone-cell of the 

 Ostrich is -roVsth of an inch, the breadth ^Vsth. 



In the thin crania of the small birds the Haversian canals are 



