CONNECTIVE TISSUE. 71 



the canaliculi. They are derived from the osteoblasts by the in- 

 corporation of the latter within the bone matrix. Besides the Ha- 

 versian system (with its lamellae) just described, there are the inter- 

 Haversian, or interstitial lamella;, and the fundamental lamella?. 

 The fundamental lamella cover the free surfaces of the bone ad- 

 jacent to the periosteum and the marrow. The canals of these lamel- 

 lae are styled Volkman's canals. 



The medulla, or marrow, occupies the central cavity. It is de- 

 rived from the osteogenetic layer of the periosteum. It is composed 

 of a connective tissue reticulum filled with cells and supplied with 

 an elaborate system of blood-vessels. The connective tissue cells, 

 or marrow cells, in young bone, become the osteoblasts; but, in old 

 bone, deteriorate into fat cells. Primary marrow is. red, but in the 

 adult bone it becomes yellow, owing to the formation of fat. The 

 marrow also contains certain large cells which are agents in the de- 

 struction of bone. They are called giant cells, osteoclasts, or my- 

 eloplaxes. They multiply by free cell-formation, and are also founl 

 in the osteogenetic layer of the periosteum. The marrow is con- 

 sidered an extension of this layer. 



Bone formation. Bone is formed by two methods centrally, 

 within the cartilage., and superficially, by the periosteum. By the 

 first method a center of ossification is produced by the transforma- 

 tion of the cartilage cells into osteoblasts. By these cells a cen- 

 tral core of bone (or bone areas) is formed. At the same time a 

 layer of bone is formed beneath the periosteum, and trabecula? are 

 thrown out from the osteogenetic layer, which extend to the center 

 of ossification and absorb the endochondral bone, thus producing a 

 central cavity for the marrow. By means of the osteoblasts the 

 permanent bone is now produced between the marrow-cavity and 

 the periosteum. Spongy bone is constituted of periosteum, a mesh- 

 work of trabeculaa, and marrow, rich in osteoblasts, etc., filling up 

 thi- spaces. 



Laboratory exercise No. 22. Bone. Harden and decalcify pieces of 

 long bone in picric acid, freeze or embed in paraffin, and stain with 

 picro-carmine or hsematoxylin, method No. 5. Examine with L. P. and 

 H. P. Make a study of the periosteum, observing the fibrous layer, con- 

 sisting of dense fibrous tissue, and the osteogenetic layer, consisting of 

 a loose fibrous reticulum rich in cells and blood vessels. Search for 



