THE CIRCULATION OF THE BONE-MARROW. 37 



3.15 p.m., ether anesthetization; posterior incision, cannula inserted. 3.25 p.m., warm physi- 

 ological salt solution started at 130 mm. Hg. 3.31 p.m., salt stopped. 3.32 p.m., warm india-ink 

 (1-4) at 85 mm. Hg. 3.39 p.m., ink stopped. 



One radius and ulna fixed in 10 per cent formalin and cleared. Marrow from opposite radius 

 and ulna fixed in Helly's fluid (Zenker-formol). Imbedded and cut in serial sections. 



In such an experimentally produced hypoplastic marrow (fig. 2) three types of 

 cells were observed, fat-cells, reticular cells, and endothelial cells. In order to 

 analyze the relations of these three cell-types the vessels of the marrow were 

 washed out with physiological salt-solution and then injected with india ink. 

 The fat-cells, together with their nuclei, were readily distinguishable and quite 

 characteristic. They were more numerous in the hypoplastic marrow, having 

 apparently replaced to a large extent the depleted cellular areas. In the fixed tissue 

 these cells appeared as empty spaces, limited by a thin but distinct membrane. 

 Each contained a more or less flattened oval nucleus, eccentrically placed and but 

 faintly stained, owing to the small amount of chromatin. Such cells made an 

 easily discernible network. Frozen sections of the fresh tissue, stained with Sudan 

 III, indicated the increased extent of these deposits of fat in the cytologically 

 depleted marrow. 



Reticular elements which conformed to all of the known criteria were to 

 be seen. They were large pentagonal or hexagonal cells with large, round, vesicular 

 nuclei ; the cytoplasm took a faint eosin stain, the nuclei showed moderate chromatin 

 content. 



The endothelial cells, in the main, conformed to certain standards and were 

 recognized through various characteristics. In the areas where the endothelium 

 could be seen lining the venules and the capillaries connecting them with arterioles 

 there was no difficulty in its identification; but there were capillaries in the bone- 

 marrow where, even after taking all the histological characteristics of endothelium 

 into consideration, certain cells could not be definitely classified. This was espe- 

 cially true in normal uninfected marrow. Unfortunately, a specific stain for 

 identifying endothelium in sections has not been developed up to the present time, 

 and such characteristics as size, morphology, and peculiarities of the nuclei are not 

 always adequate criteria. The methods developed by McJunkin (1919), Foot 

 (1921), Wislocki (1921), and others, dependent on the specific phagocytic function 

 of endothelium for various colloidal suspensions and vital dyes, were all tried in 

 the bone-marrow with indifferent success, but it is possible that additional experi- 

 ments, now being carried out, will give us at least some valuable leads in further 

 finer differential data applicable to the problem. It must not be forgotten, however, 

 that such methods depend upon direct contact between the phagocytizable par- 

 ticles and the endothelial cell; therefore, assuming that the capillaries described 

 below are probably normally non-patent to the circulating blood, we still have left 

 the need for further means of differentiating endothelium. Realizing fully, then, 

 the limitations of our present methods and the difficulties for final determination in 

 the case of a certain few individual cells, I have tried to analyze the picture presented 

 by these injections on the basis of data available at this time for their interpretation. 



