694 THE SKELETAL SYSTEM 



buds erode the bony substance with the aid of osteoclasts and produce 

 elongated channels in the bone, channels which tend to run length- 

 wise along the growing bone. Once these channels are made, osteo- 

 blasts lay down bony lamellae in concentric fashion, converting the 

 channel into an Haversian system. (Consult Maximow and Bloom, 

 '42, pp. 141-145.) The Haversian systems thus tend to run parallel 

 to the length of the bone. The Haversian canals open into the central 

 marrow cavity of the bone in some of the Haversian systems, whereas 

 others, through Volkmann's canals, open peripherally. 

 (4) While the foregoing processes are in progress, circumferential lamellae 

 are laid down around the bone. The bone's diameter thus grows by 

 the erosion of its bony walls (including previously established Haver- 

 sian systems) and by the formation of new bony substance externally 

 around the diaphysial area which is destroyed and resorbed. New 

 Haversian systems and new circumferential lamellae in this way super- 

 sede older systems and lamellae. 



At the distal ends of the bone within the spaces of the cancellous bone, 

 red marrow is found. In the shaft or diaphysis, however, the contained marrow 

 cavity is filled with yellow bone marrow, composed mainly of fat cells. 



The distal growth of elongated, cylindrically shaped bones, such as the 

 phalanges or the long bones of the limbs, is possible, while epiphyseal carti- 

 lage remains between the shaft of the bone and the bony epiphysis at the 

 end of the bone. The maintenance and growth of the epiphyseal cartilage is 

 prerequisite to the growth of these bones, for the increase in the length of 

 the bony shaft involves the conversion of cartilage nearest to the bony shaft 

 into cancellous bone. A bony cylinder of compact bone is then formed 

 around the cancellous bone. When, however, the epiphyseal cartilage ceases 

 to maintain itself, and it in turn becomes ossified, uniting the epiphysis to 

 the bony shaft, growth of the bone in the distal direction comes to an end. 

 Growth in the length of a vertebra also involves the epiphyseal cartilages 

 lying between the bony ends of the centrum and the epiphyses. Increase in 

 size of the diameter of the vertebra results from the destruction and resorption 

 of bone already formed and the deposition of compact bone around the 

 periphery. 



In the case of flattened bones of cartilaginous origin such as the scapula 

 or the pelvic-girdle bones, growth in the size of the bone is effected by the 

 conversion of peripherally situated cartilage into bone, and by the destruction 

 and resorption of bone previously formed and its synchronous replacement 

 external to the area of destruction. On the other hand, in the growth of flat 

 bones of membranous origin, the bone increases in size along its margins 

 at the expense of the connective tissue surrounding the bone. Growth in the 

 diameter of membrane bones is similar to that of cartilage bone, namely, 

 destruction, resorption, and deposition of new bone at the surface. 



