36 THE CIRCULATION OF THE BONE-MARROW. 



third more complete injection (sections 100 to 150 micra thick). Figure 5 gives 

 such a picture. In this preparation can be plainly seen what I have termed the 

 "transitional capillaries " leading directly from the arterioles to the venous sinusoids 

 and with apparently very little true arterial capillary bed. This patent capillary 

 link connecting arterioles and venules is extremely circumscribed, and it is not 

 until the venous sinusoidal anastomoses are reached that the blood spreads out in 

 lacing and interlacing vessel tufts, thence to be directed from the tuft-like branch- 

 ings into larger and larger vessels, eventually to enter the central longitudinal vein 

 almost at right angles or to find egress by way of one of the other venous outlets. 

 It will be seen that the marrow assumes almost the appearance of a segmentally 

 or lobularly divided organ, dependent upon the structural circulatory distribution 

 of these venous tufts, so completely do they ramify in definite areas, yet anasto- 

 mosing on all sides with the ramifications of bordering tufts. The relationship of 

 the arterial tree to the venous tufts on either side and the capillary transitions from 

 one to the other, even though not extensive, were easily distinguished and were 

 very characteristic in sections of injected marrow. There is little doubt, however, 

 that the extensively distributed, spacious, thin-walled venous sinusoids form nor- 

 mally the principal functioning vascular bed for the actively circulating blood in 

 the marrow; i. e., they correspond largely to the capillaries of other organs. These 

 are the vessels that have been seen and described as the fundamental units of the 

 bone-marrow by those who have worked in this field; and, while being the most 

 outstanding structures in injected marrow, by virtue of their caliber they are quite 

 as easily seen and followed in the uninjected state. By most writers they are 

 termed the venous capillaries. It would seem that venous sinus or venous 

 sinusoid might be more appropriate and desirable terminology, inasmuch as there 

 are already two types of true capillaries in the marrow, as recognized and inter- 

 preted in these observations. 



All f the vessels thus far described were plainly apparent, either grossly or 

 with the aid of the binocular microscope. The analysis of the circulation up to 

 this point had been comparatively simple through the study of injected material; 

 when an attempt was made, however, to study, under an oil-immersion lens, the 

 detailed ramifications of the smaller vessels and the extent and continuity of the 

 individual endothelial cell distribution, difficulties were at once encountered. 

 It was found that analysis of these finer points in normal marrow is extremely 

 unsatisfactory, if not quite impracticable. In order to analyze with any certainty 

 the finer ramifications of the vascular pattern, i. e., the cytological relationships, 

 it is essential, in the first instance at least, to have a marrow depleted as far as 

 possible of all the free cells. An attempt was therefore made to produce experi- 

 mentally a hypoplastic bone-marrow in the pigeon. The desired condition was 

 secured through simple starvation for periods varying from 10 to 18 days. 



Protocol, Pigeon 19 A. 



January 29. Pigeon in excellent condition, weight 475 grams. Diet restricted to fresh water 

 every morning. Condition remained excellent up to February 7. February 10, conditio!! good. 

 February 15, pigeon in fair condition but emaciated; weight 340 grams. Operation same date. 



