BONE STRUCTURE. 



717 



granular structure can be seen. p§| 

 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 lamina?, the bone- 

 cells and their canaliculi ; 

 even these, except the bony 

 laminre, 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 

 skull, and in the irregular 

 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 



Fig. 350.—^ portion of the Cranium 

 of a Siren (Siren laeertina), re- 

 markable for the large size of the 

 bone-cells and of the canuliculi, 

 which are larger in this animal than 

 in any other yet examined. As in 

 the preceding specimen, no Havei - 

 sian canals are present. 



of the skull is 

 to include the 



Pig. 351.-^1 small portion of bone 

 mad.6 SO as taken from the exterior of the shaft 

 rlirpo ln-1701'Q of the Humerus of a Pterodactyls ; 

 i/iiieu lay tub this exhibits the elongated bona- 



above mentioned, bone-Cells cells characteristic of the order 



may be seen in all ; but each eph ia " 



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



