202 HUMAN PHYSIOLOGY 



2. Bone Marrow. The structure of bone marrow is con- 

 sidered below, but it may be stated here that young leuco- 

 cytes or leucoblasts, in the condition of mitosis, are abundant, 

 and that they seem to pass away in the blood stream. They 

 are of all varieties. In certain pathological conditions the 

 formation of these cells is increased and a leucocytosis 

 results (Fig. 103). 



II. Erythrocytes. These are formed after birth in the 

 red marrow of bone. Marrow consists of a fine fibrous 

 tissue with large blood capillaries or sinuses running in it. 

 In the fibrous tissues are numerous fat cells, and generally 



FIG. 102. Section of a Lymph Gland, a., capsule ; 6., germ centres of cortex ; 

 c., sinuses ; d. , trabecula ; e. , germ centres of medulla. 



a considerable number of multi-nucleated giant cells. In 

 addition to these are the young leucocytes, leucoblasts, which 

 used to be called the proper cells of the bone marrow, and 

 lastly young nucleated red cells, the erythroblasts. After 

 haemorrhage, the formation of these becomes unusually 

 active, and may implicate parts of the marrow not generally 

 concerned in the process, and hence the red marrow may 

 spread from the ends of the long bones, where it is usually 

 situated, towards the middle of the shaft. The nuclei of the 

 erythroblasts atrophy or are shed and the cells escape into 

 the blood stream. The red marrow has the power of re- 

 taining the iron of disintegrate'd erythrocytes, which are 



