40 NORMAL HISTOLOGY. 



shaft of the long bones or the irregular interstices between the trabeculae of 

 the cancellated tissue, are filled with bone-marrow. The latter also extends 

 into the larger Haversian canals. Apart from its fundamental relations to the 

 development of bone, in addition to supporting the medullary blood-vessels, 

 and therefore assisting in maintaining the nutrition of the bone, the marrow 

 plays a very important role in connection with the production of blood-cells. 

 Indeed, with its chief functions in mind, bone-marrow is classed as a blood- 

 forming organ, and, as such, finds it's systematic consideration with the 

 blood. As a matter of convenience, however, a general description of the 

 marrow-tissue is here given, while the relations of its cellular elements to 

 the circulation are discussed in connection with the Blood (page 100). 



Although of a reddish tint within all the bones of the early skeleton, the 

 marrow in the adult includes two kinds the red and the yellow. Thus, 

 within the shaft of the long bones it appears as a light yellowish tissue, pre- 

 senting the characteristics of ordinary adipose tissue; while within the upper 

 ends of the humerus and of the femur, and especially within the bodies of 

 the vertebrae, the ribs, the sternum and the diploe of the cranium, the marrow 

 possesses a dull red color. 



Red Marrow. The ingrowth of the periosteal tissue and blood-vessels 

 constitutes the primary marrow of the foetal skeleton; from this tissue the 

 red marrow filling the young bones is directly derived. The red marrow is, 

 therefore, the first formed and typical variety. After early childhood, how- 

 ever, the marrow within the bones of the extremities suffers gradual invasion 

 by fat, until, with the exception of the marrow within the upper ends of the 

 long bones, as the femur, the red tissue gives way to the yellow, the fat- 

 cells replacing most of the marrow-elements. 



When examined in section after fixation and appropriate staining, the 

 red marrow exhibits a delicate connective tissue reticulum which supports 

 the blood-vessels and contains within its meshes numerous cells (Fig. 55). 

 Next the bone, the fibrous tissue forms a thin membrane, the endosteum> 

 lining the medullary cavity and extending into the larger Haversian canals. 

 The more characteristic cells encountered within the red marrow include: 

 ( i )the myelocytes, very numerous and of different ages; (2) the eosinophile s ; 

 (3) the megakaryocytes or giant cells and (4) the erythroblasts and other 

 stages of red blood-cells. In addition lymphocytes, connective tissue cells, 

 fat-cells, mast-cells and osteoclasts are usually present in small and varying 

 numbers. 



The myelocytes include three varieties of cells which, while differing 

 in the granularity of their cytoplasm 'and the form of their nuclei, are 

 probably directly genetically related, (a) The myeloblasts are relatively large 

 ovoid cells, with round nuclei and cytoplasm devoid of granules. They are 

 few in number and regarded as the parent elements (premyelocytes). (b) 

 The myelocytes, the most numerous of the marrow-cells, are of uncertain 

 form and possess large round nuclei and cytoplasm containing fine neutro- 

 philic granules. (*:) The polymorphonudear leucocytes, the descendants of 

 the preceding elements, are somewhat smaller and more granular than the 

 myelocytes and distinguished by the lobulated nuclei so conspicuous in the 

 most common form of colorless blood-cells. If the genetic relations here 

 outlined be accepted, the typical leucocytes represent later generations of 

 the marrow-cells, which thus become important contributors to the blood. 

 The eosinophiles, sparingly represented, are conspicuous by reason of the 

 coarse granules within their cytoplasm which color intensely with acid stains 

 (eosin). The relation of the eosinophiles to the myelocytes shares the 



