BONE STRUCTURE. 



717 



granular structure can be seen. 

 The parts which a transverse 

 or a longitudinal section of a 

 long bone of a mammalian 

 animal exhibits, will be the 

 Haversian canals, the con- 

 centric bony laminae, the bone- 

 cells and their canaliculi; 

 even these, except the bony 

 lamina, may be seen in all 

 mammalian bones (fig. 345). 

 Whether long or otherwise, 

 they are, nevertheless, so dif- 

 ferently arranged in the flat 

 bones, such as those of the 



i -I! j ,-i . i Fig. 350. A portion 



skull, and in the irregular */ a siren (siren 

 bones, as the vertebrae, as to 

 require notice. Those of the 

 head are composed of two thin 

 layers of compact texture; 

 enclosed between which is 

 another layer of variable thick- 

 ness, of a cellular or cancel- 

 lated structure. 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 brittle- 

 ness, is termed by anatomists 



the vitreous table. When 



a vertical section of a bone Flg ^_ A ^ ^ 



of the skull IS made SO as taken from the exterior of the shaft 

 +rk iT^lml thp. tlirpp Iflvpro of the Humerus of a Pterodactyl* ; 



to include tne tnree layers this exhibit3 th6 elongated bone- 

 above mentioned, bone-Cells cells characteristic of the order 



may be seen in all ; but each Reptaia ' 



of the three layers differ in structure : the middle or can- 



which are larger in this animal than 



