l8 THE ELEMENTARY TISSUES 



lamellae do not fill. The Haversian canals are minute longi- 

 tudinal channels, each surrounded by its lamellae within which 

 run still smaller longitudinal channels, called lacunae. Connect- 

 ing the main channel and the lacunae, and radiating in all direc- 

 tions 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 cartilage. It 

 consists of two layers, an outer fibrous and an inner fibro-elastic 

 layer. However, during the period of development a third 

 layer, the osteogenetic layer, lies to the interior. 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 formation 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 contains 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 varieties of cells, (b) the eosinophiles, 

 which are few in number, but which are conspicuous by the 

 presence of coarse granules within the cytoplasm, which are 



