32 THE CIRCULATION OF THE BONE-MARROW. 



region of which he also believed the endothelium of the sinuses to be lacking. 

 Brinckerhoff and Tyzzer (1902), in studies on the uninjected marrow of rabbits, 

 described places in which the blood-stream is not confined within endothelial walls 

 but wanders through channels in the reticulum and the masses of cells. More 

 recently, Bunting (1919) describes the marrow vascularization as follows: 



"The circulation as revealed by natural injections of the rabbit's marrow is unlike 

 that of any other organ but resembles superficially that of the spleen pulp." 1 



He further states that there is no capillary network and describes slender 

 arterioles originating near the center of the marrow and proceeding, without 

 capillary side branches or anastomoses, to the periphery, where they open directly 

 into wide, thin-walled sinuses. 



Desiring to investigate the relationship which endothelium might bear to 

 the supply of red blood-cells in the mature organism, it became necessary to know 

 its distribution at first hand. The interesting results which have attended these 

 studies are presented with the belief that they open up a new field of possibilities, 

 only vaguely hinted at heretofore, but now having a definite basis in anatomical 



' MATERIALS AND METHOD. 



The conclusions reached in this paper are based largely on a series of investi- 

 gations on about forty adult pigeons. Further experiments of a similar character, 

 conducted on the dog, cat, rabbit, and white rat, seem to substantiate and cor- 

 roborate the gross findings in the pigeon, so far as I have been able to observe in a 

 limited series. A larger number of observations on mammals will be necessary 

 before a complete report can be made. 



An attempt has been made to get complete injections of the vascular system 

 of the bone-marrow. This has not been easy, the difficulties being fourfold: 

 (1) to secure a satisfactory medium for injection, (2) to keep the pigeons alive suffi- 

 ciently long during the preliminary insertion of the cannula, etc., (3) to secure and 

 maintain just the right pressure for perfusing, and (4) to wash out and inject 

 under conditions as nearly physiological as possible and for the optimum length 

 of time. 



It has been found, in general, that pigeons are peculiarly susceptible to 

 chloroform. All operations have been done on anesthetized birds, and a light 

 ether anesthetization has been found entirely satisfactory. It is desirable to have 

 the animal alive during the first stage of the washing-out process. 



My most successful injections were made with a pressure of 130 mm. of mercury 

 for both saline and ink. When the pressure was materially increased above this 

 point, rupture and extravasation frequently occurred, whereas with pressures 

 below this level an incomplete injection was apt to result. Both the injection 

 material and the physiological saline were previously warmed to a degree somewhat 

 above body-temperature to insure their reaching the vessels at body-temperature. 

 With a free flow this saline should not be run longer than 8 minutes, preferably a 

 shorter period, judging by the clarity of the venus outflow. The injection mass 



1 Mollier (1909) has demonstrated openings into the splenic pulp, i. e., fenestrated vessel-walls. 



