BONE STRUCTURE. 



717 



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 flat 

 bones, such as those of the . 



, ,, ' , ., . i Fig. 350. A portion of the Cranium 



Skull, and in the irregular O f a Siren (Siren lacertina), re- 

 l->nnp<3 a<5 tVip vprtphriP a<? to markable for the large size of the 



Dones, as tne verteorae, as to bone . cellg and of t g e canulicu ii 

 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 ^_ A ^ ^.^ of ^ 



Of the Skull IS made SO as taken from the exterior oj s the shaft 

 frv irmlnrlo -Hio +Virna lavpra of the Humerus of a Pterodactyle ; 



to include tne tnree layeis thig exhibit3 tl?6 elongated bone- 

 above mentioned, bone-cells cells characteristic of the order 

 may be seen in all ; but each Reptil " 

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



