STRUCTURE OF BONE. 



585 



sometimes be seen in very thin natural plates of bone, sucb as in 

 that forming the scapula (shoulder-blade) of a Mouse ; but they 

 are displayed more perfectly by artificial sections, the details of 

 the arrangement being dependent upon the nature of the speci- 

 men selected, and the direction in which the section is made. 

 Thus when the shaft of a "long" bone of a Bird or Mammal is 

 cut across in the middle of its length, we find it to consist of a 

 hollow cylinder of dense bone, surrounding a cavity which is 

 occupied by an oily marrow ; but if the section be made nearer 

 its extremity, we find the outside wall gradually becoming thinner, 

 whilst the interior, instead of forming one large cavity, is divided 

 into a vast number of small chambers or cancelli, which com- 

 municate with each other 



and with the cavity of the FIG. 299. 



shaft, and are filled, like it, 

 with marrow. In the bones 

 of Eeptiles and Fishes, on 

 the other hand, this " cancel- 

 lated" structure usually ex- 

 tends throughout the shaft, 

 which is not so completely 

 differentiated into solid bone 

 and medullary cavity, as it 

 is in the higher Vertebrata. 

 In the most developed kinds 

 of "flat" bones, again, such 

 as those of the head, we find 

 the two surfaces to be com- 

 posed of dense plates of bone, 

 with a " cancellated" struc- 

 ture between them ; whilst 

 in the less perfect type pre- 

 sented to us in the lower 

 Vertebrata, the whole thick- 

 ness is usually more or less 

 " cancellated," that is, burrowed out by medullary cavities. 

 When we examine, under a low magnifying power> a longitu- 

 dinal section of a u long" bone, or a section of a "flat" bone 

 parallel to its surface, we find it traversed by numerous canals, 

 termed Haversian after their discoverer Havers, which are in 

 connection with the central cavity, and are filled, like it, with 

 marrow: in the shafts of long bones, these canals usually run in 

 the direction of their length, but are connected here and there 

 by cross branches ; whilst in the flat bones, they form an irregu- 

 lar network. On applying a higher magnifying power to a thin 

 transverse section of a long bone, we observe that each of the 

 canals whose orifices present themselves in the field of view (Fig. 

 299), is the centre of a rod of bony tissue (1), usually more or less 

 circular in its form, which is arranged around it in concentric 



Minute structure of Bone, as seen in transverse sec- 

 tion : 1, an ossicle surrounding an Haversian canal, 

 3, showing the concentric arrangement of the lamellae; 

 2, the same, with the lacunae and canaliculi ; 4, por- 

 tions of the lamella? parallel with the external surface. 



