'.•I 



HISTOLOGY OF THE FROG 



127 



compact bone, which is very firm and dense, and spo?igy or 

 cancellous bone, which is made up of plates and bars forming 

 a structure which is comparatively loose and lacking in 

 strength. The latter is found within the center of the verte- 

 brae and to a small extent within some of the long bones. A 

 good example of 

 compact bone 

 may be obtained 

 by making a cross 

 section of the fe- 

 mur. The central 

 part of the bone is 

 hollow and filled c--/"> '■ _y " : " * ';* ''^ ^ / 5*S« ?W 



with marrow, and ^ ' * / * ' , „"' 



the outer surface V ^ *• ••■■" ' &***?^\ 



is covered by a , • . ' ' , , — ' - 



layer of perioste- , ''- '*' ■ 



urn, which is simi- ** / 



lar in structure to -' ~s 



the pericondrium Fig. 34. — A part of a cross section of the femur of 

 surrounding the thefro ^ '.«*■««*! 

 cartilage. The 

 bony substance is arranged in concentric layers, or lamella, 

 which contain numerous lacuna, in which lie the bone cells. 

 From the lacunae fine branching tubes, or canaliculi, contain- 

 ing processes from the bone cells, are given off which extend 

 in all directions and anastomose with the canaliculi of neigh- 

 boring spaces. 



Bones increase in thickness by the addition of successive 

 layers to the outside. The osteoblasts, or cells forming the 

 inner layer of the periosteum, give rise continually to new 

 bone cells which cause the deposition of new layers of bony 

 substances between the periosteum and the old bone. New 



lc, lacunae; Im, lamellae; 

 (After Parker and Parker.) 



