BONE STRUCTURE, 



17 



Fig. 350. — A portion of the Cranium 

 of a Siren (Siren lacertina), re- 

 markable for the large size of the 

 hone-cells and of the canuliculi, 

 which are larger in this animal than 

 in any other yet examined. As in 

 the ijreceding 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 exliibits, will be the 

 Haversian canals, the con- 

 centric bony laminse, the bone- 

 cells and their canaliculi ; 

 even these, except the bony 

 laminae, mav 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 vertebrte, 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- 

 ne«s, 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 dijDloe : 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 

 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 can- 



Fig. 351.—^ small portion of bone 

 taken from the exterior of the shaft 

 of the Humerus of a Pterodactyle ; 

 this exhibits the elongated bone- 

 cells characteristic of the order 

 Rejjtilia. 



