1 8 THE ELEMENTARY TISSUES 



fill in the spaces which the other lamellae do not fill. The 

 Haversian canals are minute longitudinal channels, each sur- 

 rounded by its lamellae within which run still smaller longi- 

 tudinal channels, called lacunas. Connecting the main chan- 

 nel and the lacunae, and radiating in all directions between 

 them are other very minute channels known as canaliculi. 

 Each Haversian canal with its surrounding lamellae, lacunae 

 and canaliculi composes an Haversian system. 



A fibrous membrane, the periosteum, forms the outer 

 covering of all bones except when they are covered with car- 

 tilage. It consists of two layers, an outer fibrous and an in- 

 ner fibre-elastic layer. However, during the period of devel- 

 opment a third layer, the osteogenetic layer, lies to the inte- 

 rior. It possesses a rich blood supply which nourishes the 

 subjacent bone, and contains cells which later become bone- 

 forming elements the osteoblasts. 



Bone marrow is the highly vascular substance found 

 within the central cavity of the long bones and the Haversian 

 canals. It may be divided into two classes: (i) Red bone 

 marrow, and (2) yellow marrow. In early childhood all the 

 marrow in the bones is red or has a reddish tint, but in adult 

 life we find two kinds the red and the yellow. 



( i ) Red bone marrow is classed as one of the blood- 

 forming organs since it plays an important role in the for- 

 mation of the blood. When stained and examined under the 

 microscope it is found to consist of a delicate connective- 

 tissue reticulum which supports the blood-vessels and con- 

 tains in its meshes numerous cells. On the outside of the 

 marrow, next to the bone, we find a thin fibrous-tissue coat, 

 the endosteum, which lines the medullary cavity and extends 

 into the larger Haversian canals. The more numerous of 

 the cells found in the red marrow are, (a) the myelocytes, 

 which are very numerous and contain several different va- 

 rieties of granules, (b) the eoslnophiles, which are few in 

 number, but which are conspicuous by the presence of coarse 

 granules within the cytoplasm, which are colored intensely 



