HISTOLOGY OF THE FROG 105 



canals are occupied during life by blood-vessels an arteriole 

 and a venule to each and a small amount of connective 

 tissue containing branched cells. Some of the larger canals 

 also contain marrow. Each of the lacunae contains a small, 

 nucleated, branched cell or bone-corpuscle, which com- 



Fig. 21. 



Portion of a transverse section through the femur of a pig to show the 

 structure of bone. The section shows two concentric Haversian 

 systems, each with its central Haversian canal, surrounded by 

 numerous lacunae with their canaliculi. At the top of the figure 

 the external or periosteal layer is shown, in which the lacunae are 

 elongated in a direction parallel with the surface of the bone. 



municates by means of protoplasmic processes running 

 through the canaliculi with the processes of adjacent 

 corpuscles, and eventually with the branched connective 

 tissue cells of the Haversian canal. The outermosjt canaliculi 

 of a Haversian system do not, as a rule, communicate with 

 those of an adjoining system, but bend round to join the 

 peripheral lacunae of their own system. The elaborate system 

 of blood-vessels and nucleated cells with communicating 

 protoplasmic processes shows that the nutrition of bone is 

 dependent on the presence of living protoplasm, and the whole 

 structure of bone may be compared to that of connective 

 tissue, the gelatin plus lime salts forming the matrix, the bone- 



