MAREOW CELLS 171 



marrow cells. According to the relative proportion of these ele- 

 ments marrow is said to present two types, the yellow and the red 

 marrow. The yellow marrow consists almost entirely of fat, with 

 only occasional bands of true marrow tissue. The red marrow 

 contains very little fat, but is so abundantly supplied with blood 

 and marrow cells as to closely resemble a very vascular lymphoid 

 tissue. The embryonic medulla of all bones contains fetal red 

 marrow, but in later life the larger masses in the medulla of the 

 shafts of the long bones is, in man, changed to the yellow variety. 

 The red marrow, however, persists in the epiphyses of the long 

 bones and in cancellous bone generally ; it is especially character- 

 istic of the marrow cavities of the ribs, vertebrae, base of the skull, 

 and sternum. 



Red mar r oio consists of fibrous and reticular tissues which are 

 infiltrated by marrow cells and richly supplied with small blood 

 vessels. The smaller veins possess exceedingly thin walls ; in fact, 

 these are so delicate that it is almost impossible to determine 

 whether or not their endothelium, as also that of the capillaries, 

 maybe occasionally absent, thus placing the blood stream in direct 

 communication with the pulp of the bone marrow. 



The following types of MAREOW CEILS, together with the 

 fibrous connective tissue, reticulum, and blood vessels, make up 

 the structure of red marrow (Figs. 158 and 164). 



1. Myelocytes (Marrow Cells Prefer) .These cells closely re- 

 semble the lymphatic corpuscles and the leucocytes of the blood, 

 and, like them, include several cell types : 



A. Lymphocytes; small cells with an ovoid deeply staining 

 nucleus, and a very narrow rim of non-granular, faintly basophilic 

 cytoplasm. 



B. Large mononuclear. marrow cells ; whose ovoid nucleus 

 stains faintly and is sometimes indented or constricted; their 

 broad rim of non-granular cytoplasm is slightly stained by basic 

 dyes. Occasionally the cytoplasm contains a scanty supply of very 

 fine granules ; rarely, also, the nucleus consists of two ovoid lobes 

 which are united by a coarse chromatin filament (Jolly*). 



C. Poli/nuclear neutrophile marrow cells ; the nucleus of these 

 cells is polymorphous and stains deeply ; their cytoplasm forms a 

 broad rim in which are many fine neutrophile granules. The cells 

 of this variety, as well as those of the preceding, have been fre- 

 quently found to possess centrosomes and mitotic figures. 



* Arch, d'anat. rnic., 1900. 



