578 



THE MICROSCOPE. 



stance ? 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 which a transverse or a longitudinal section 

 of a long bone of a mammalian animal exhibits, will be 

 the Haversian canals, the concentric bony laminae, the 

 bone-cells and their canaliculi ; even these, except the bony 

 laminae, may be seen in all mammalian bones. (Fig. 315.) 

 Whether long or otherwise, 

 they are, nevertheless, so dif- 

 ferently arranged in the flat 

 bones, such as those of the 

 skull, and in the irregular 

 bones, as the vertebrae, as to 

 require a short description at 

 this stage of our inquiry. 



" The bones of the cranium 

 are in all cases composed of 

 two thin layers of compact 

 texture ; enclosed between 

 which is another layer of vari- 

 able thickness, of a cellular or 

 cancellated structure. The two 

 outer layers are called tables, 



Fig. 321. A small portion of bone , J , . ,-, 



taken from the exterior of the shaft the OU6 being the OUter, 



of the Humerus of a Pterodactyle ; V, P ntViPr fhp irmpr taVilp fmH 



this exhibits the elongated bone- ttie Ottier tne 



cells characteristic of the order 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 differ in 

 structure : the middle or cancellated structure will be found 

 to resemble the cancellated structure in the long bones, viz. 

 thin plates of bone, with one layer of bone-cells without 

 Haversian canals j the outer layer will exhibit Haversian 

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



