BONE STRUCTURE. 



717 



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 Haver- 

 sian canals are present. 



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 

 laminae, may be seen in all 

 mammalian bones (fig. 345). 

 Whether long or otherwise, 

 they are, nevertheless, so dif- 

 ferently arranged in the fiat 

 bones, such as those of the _ 



-, ,, j ,v • i Fig- i*60. — a portion of the Cranium 



skull, and m the irregular / a siren (Siren lacertina), re- 

 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. 351.-^4 small portion of bone 



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



tn iTiplnrlp thp thrpp lavprs of the Humerus of a Pterodactyle ; 

 tO memae lue inree id^eib this exhibits the elongated bone - 



above mentioned, bone-Cells cells characteristic of the order 

  • n -l "l Reptilia. 



may be seen in all ; but each 



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



