40 THE CIRCULATION OF THE BONE-MARROW. 



medullary parenchyma could be seen in the same areas without any attached 

 granules of ink. (4) The walls of the veins and venules appeared as continuous 

 endothelium-lined channels, similar in appearance to the vascular bed elsewhere 

 in the body, but with conical openings into the tiny capillary network. (5) Finally, 

 I have obtained relatively complete injections of these very fine, extensive, lace- 

 like vessels without the slightest evidence of any of the injected particles outside 

 the closed channels in the parenchyma. In other words, in the adult bone-marrow 

 here studied, there was no evidence of any fenestrated vessel-wall, similar to 

 that described by Mollier (1909) for the spleen. One need only contrast a true 

 extravasation with one of these injections to recognize the difference at once. 

 It is very possible, however, that in an injection of normal bone-marrow that 

 filled only arterioles, transition capillaries, and venous sinusoids these conical 

 capillary projections might be interpreted as fenestrated openings. 



The endothelial cells of the inter-sinusoidal capillaries were thinned out, in 

 contrast to their number and arrangement in a larger vessel, and in many instances 

 had been forced apparently into the interstices between encroaching fat-cells and 

 looked more nearly like primitive embryonic endothelium. They could, neverthe- 

 less, be seen to line these spaces through which granules of the injected fluid had 

 been forced. The picture then was that of a very extensive capillary bed which 

 simulated, in the appearance, distribution, and arrangement of its vessels and 

 cell elements, an embryonic plexus rather than the ordinary mature capillary 

 plexuses elsewhere recognized in the adult. This plexus was fined everywhere by 

 intact endothelium. 



. It may now be possible to bring out, clearly and definitely, the really striking 

 contrast between the type of circulation to be found in the spleen and that inherent 

 in the bone-marrow. There has been, in the past, a tendency to draw analogies 

 between the two circulations. This, we feel, is quite unjustified, both from the 

 standpoint of the function and from the very different nature of the two structures. 

 Mall (1902, 1903) showed, in a final and crucial experiment, that the spleen was 

 adapted to an easy, rapid, and complete emptying of its blood-content at any 

 given moment. He tied all of the splenic veins in a dog, under ether, and let the 

 arteries fill the spleen with blood to its maximum distention ; he then cut the liga- 

 tures from the veins and watched the speedy contraction of the organ, and proved 

 by frozen sections that the pulp, which had been engorged with red cells, became 

 totally empty in a few seconds. This could be possible only in case the entire splenic 

 pulp were to be regarded as a peculiar capillary bed in very free communication 

 with its efferent veins. The demonstration of the fenestrated endothelial lining 

 of the veins of the splenic pulp by Mollier (1909) completed the understanding 

 of this special type of circulation. The well-known bands of smooth muscle in the 

 trabecular are accessory structures peculiar to this system. The spleen is there- 

 fore a contractile organ, capable of emptying itself at intervals, and thus providing 

 a means of propelling the whole blood, which has free access to the interstitial 

 tissues, back into and through the general circulation. In contrast to this, the 

 venous sinuses of the bone-marrow have an intact endothelial wall; the inter- 



