BONES. 265 



(a) Bone-marrow. 



In all bones of higher animals we find a bone-marrow. In 

 the long bones this fills the axial cavity and enters the larger 

 Haversian canals. In the flat bones, on the contrary, it fills 

 up the meshes of the spongy substance. Two kinds of bone- 

 marrow can be distinguished : red and yellow marrow. The 

 first is found in all bones of embryos and young individuals. 

 In the course of time it changes in some bones (e. g., the 

 diaphyses of the long and short bones of the extremities) into 

 yellow marrow. Only in the epiphyses of these bones, in the 

 bodies of vertebrae, and in the flat bones, is there found red 

 marrow in the adult. 



The red marrow is a lymphoid organ which is the main 

 place of formation of the red blood-cells. The different ele- 

 ments contained in the red marrow are the following 

 (Fig. 197): 



1. Myelocytes. These are somewhat similar to some kinds 

 of leucocytes. In normal blood they are not found, while in 

 leukaemia they are very abundant. Their nuclei are very 

 large, sometimes lobed, and surrounded by a more or less finely 

 granular protoplasm. The nuclei stain faintly, and the proto- 

 plasm is sometimes quite abundant. 



2. Nucleated Red Blood-corpuscles. The protoplasm is col- 

 ored yellow on account of the haemoglobin present. The 

 nucleus usually is placed excentrically and stains very deeply. 

 These cells are known also as erythroblasts or normoblasts, 

 since they are the forerunners of the erythrocytes. They vary 

 considerably in size, very large ones being known as raegalo- 

 blasts, and small ones as microblasts. These unusual forms 

 occur, however, more often in certain diseases. 



3. Non-nucleated Red Blood-corpuscles. These are derived 

 from the nucleated red corpuscles. 



4. Giant Cells. These are probably modified leucocytes. 

 They contain one or more nuclei, whose form maybe round, 

 lobed, or annular. The old theory that the multinucleated 

 giant cells arise by a fusion of many cells is abandoned. They 



