BONE. 383 



canals, the concentric bony laminae, the bone-cells and their canaliculi ; 

 although all these parts, except the bony laminae, may be seen in all 

 mammalian bones. Whether long or otherwise, they are, nevertheless, 

 so differently arranged in the flat bones, such as those of the skull, and 

 in the irregular bones, such as the vertebra, as to require a short de- 

 scription at this stage of our inquiry. 



The bones of the cranium are in all cases composed of two thin 

 layers of compact texture, which enclose another layer of variable thick- 

 ness, which is cellular or cancellated. The two outer layers are called 

 tables. the one being the outer, the other the inner table j and the 

 middle or cancellated layer is termed the diploe : in this last the prin- 

 cipal blood-vessels ramify. The outer table of the skull is less dense 

 than the inner ; the latter, from its brittleness, is termed by anatomists 

 the vitreous table. When a vertical section of a bone of the skull is 

 made so as to include the three layers above mentioned, bone-cells may 

 be seen in all ; but each of the three layers will differ in structure : the 

 middle or cancellated structure will be found to resemble the cancel- 

 lated structure in the long bones, viz. thin plates of bone, with one 

 layer of bone-cells without Haversian canals ; the outer layer will ex- 

 hibit Haversian canals of large size, with bone-cells of large size, and a 

 slightly laminated arrangement ; but the inner or vitreous layer will 

 be found to resemble 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, 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 1 -2000th of an inch, the breadth 1 -6000th. 



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 extremities are either hollow or cancellated ; the ribs 

 of the serpent tribe are hollow, the medullary cavity performing the 



