BONE. 



57 



osteum, parallel with the surface of the bone, the periostea! lamellae [outer 

 circumferential or outer ground lamellae]. If the bone is hollow, having a 

 marrow cavity, similar lamellae may be deposited over the inner surface of 

 the shaft by a formative layer called the endostcum. These lamellae are cud- 

 osteal lamellae [inner ground or circumferential lamellae, marrow lamellae] 

 and they line the marrow cavity. The four sets of lamellae are shown in 

 Fig. 62. 



Lamellar bone is compact, differing notably from the spongy network 

 of trabeculae seen in the embryo. Compact bone is found in the outer parts 

 of the long and flat bones and as a thin outer layer in short bones. Spongy 

 bone is found in the interior of long bones, and of flat bones (where it is 

 called diploe), and it constitutes the greater part of short bones and epi- 

 physes. It is due in part to the persistence of the embryonic trabeculae, 

 and in part to the reduction 

 of compact bone to slender 

 spicules through processes 

 of absorption. Scarcely has 

 bone formed before portions 

 of it begin to be resorbed. 

 The osteoblasts disappear 

 locally and in place of them 

 there are large irregular 

 masses of protoplasm con- 



h 



taining several separate nu- 

 clei. The idea that these 

 structures arise by the fusion 

 of several osteoblasts is not 

 accepted; the nuclei are thought to arise by repeated division w r ithin a 

 mass of protoplasm which enlarges but does not divide. The form of 

 giant cell resulting is called an osteoclast, from its supposed function of 

 destroying bone. The osteoclasts, Fig. 60, are often seen in hollows 

 which they are thought to have excavated in the ground substance, and 

 which are called Howship's lacunae. There seems to be no satisfactory 

 evidence that the osteoclasts are the cause rather than a product of those 

 conditions which lead to the dissolution of bone. The process of resorp- 

 tion is of the greatest importance, since it prevents bones from becoming 

 solid and heavy. While new bone is forming on the periosteal surface, 

 old bone is being dissolved, both around the marrow cavity and in the 

 deeper Haversian canals. This process produces most of the spongy bone 

 of the adult. 



Reviewing the preceding paragraphs, it may be said that bone appears 

 first as strands of ground substance produced by osteoblasts derived from 



Endosteal 

 lamellae. 



Marrow. 



FIG. 62.- 



FKOM A CROSS SECTION OF A METACARPAL OF MAX. 



X 50. 

 Resorption line at h. 



