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If we consider what has been already mentioned as entering into 

 the composition of a bone, viz., the medullary cavity, the Haversian 

 canals, the canaliculi, and the bone-cells, we shall find that every part 

 thus described has been more or less hollow ; where, then, is the true 

 bony substance ? This is no other than the small granules of ossific 

 matter which are situated between the canaliculi of the bone-cells, 

 each granule having an investment of soft animal matter, by which 

 the whole mass of granules is kept in firm apposition. 



The parts, then, which a transverse or a longitudinal section of a 

 long bone of a mammalian animal will exhibit, will be the Haversian 

 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 vertebrae, as to require a short 

 description at this stage of our enquiry. 



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

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

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

 called tables, the one being the outer, the other the inner table, and 

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

 principal 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 re- 

 semble the cancellated structure in the long bones, viz., thin plates of 

 bone, with one layer of bone-cells without Haversian canals ; the 

 outer layer will exhibit 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 ex- 

 hibit both smaller Haversian canals and more numerous bone-cells of 

 ordinary shape around them. The same thing will hold good of the 

 ribs, scapula, and os innominatum, and of the irregular bones, such as 

 the vertebrae, the small bones of the wrist, and ankle-joints, they being, 

 in fact, nothing more than a cancellated or spongy tissue, enclosed by 

 a compact layer of denser bone. It will be noticed that the bone- 

 cells in all these bones are less regular in their size than in the long 

 bones, and hence there will be required more caution before pro- 



