42 THE CIRCULATION OF THE BONE-MARROW. 



DISCUSSION. 



The question that immediately presents itself is that of the function of this 

 vast bed of endothelium extending throughout the bone-marrow, which, as far as 

 can be determined, does not function as a channel for the active circulating blood- 

 stream, at least not normally and regularly. In the absence of full experimental 

 evidence, it is natural and helpful for one to reason by analogy in an attempt to 

 secure working hypotheses in explanation of the phenomena not at present fully 

 understood. This is not without full comprehension of the very great difficulty 

 of following such a line of reasoning without the possibility of grave error. 



Richards (1922) has recently reported observations on the glomerular activity 

 in the frog's kidney. He believes that the majority of the glomerular capillaries 

 are not continuously functioning actively, but that there are intervals during 

 which the individual glomerular capillary is closed to the main blood-current. 

 It is possible that could the hemopoietic tissues be examined directly and as satis- 

 factorily as has been done in the case of the frog's kidney by Richards, a similar 

 phenomenon in the marrow capillaries would be found. 



Krogh (1918, 1919) has published some most illuminating observations on the 

 capillary circulation in the muscle of the frog and guinea-pig. He finds that 

 in resting muscle most of the capillaries are in a state of contraction and closed 

 to the passage of blood. It was impossible to inject, even under high pressures, 

 any but the few functioning capillaries that were patent at the moment; but by 

 tetanic stimulation, with gentle massage, or in spontaneously contracting muscles 

 a large number of capillaries were opened up and were subsequently observed to 

 contract again. He found the average diameter of open capillaries in resting muscle 

 to be much less than the dimensions of the red corpuscles which become greatly 

 deformed during their passage. Finally, he has shown and called attention to the 

 important fact that clinical hypersemia and anaemia are due mainly to changes in 

 the caliber and number of open capillaries, and that the capillaries are not merely 

 passively dilated by blood pressure but are controlled by a " capillario-motor 

 system" independent of the "arteriomotor system." 



It was only through ingenious pressure injections that capillary channels, long 

 suspected but often denied, were finally demonstrated in the valves of the heart 

 by Bay ne- Jones (1917). It is conceivable that under certain physiological con- 

 ditions they may be more obviously patent. Rich (1921), in experiments on the 

 omentum, has shown, both by induced inflammation and by histamin injection, a 

 capillary bed much increased over that seen in the normal omentum, demonstrating 

 the large number of ordinarily non-patent, occult vessels capable of responding 

 and functioning protectively when occasion demands. Lee (1922), in some investi- 

 gations on lymphatic circulation following the ligation of the thoracic duct, de- 

 scribed a most interesting phenomenon. Within 10 days after the careful com- 

 plete ligation of the thoracic duct he found a most extensive anastamotic distribu- 

 tion of fine lymphatic vessels spreading out along the wall of the aorta, and eventu- 

 ally (within two weeks) a completely compensated, equilibrated lymphatic circula- 

 tion was established. It seems probable that these may be preexisting collapsed 



