ARTICLES 275 



excluding passive effects due to changes in the arterioles. A 

 dilatation in these vessels would raise the pressure at the 

 place where they become capillaries, and naturally distend 

 these to some extent. Lister in 1858 described dilatation of 

 the capillaries in the frog's web under the action of chloroform 

 and other agents. Roy and Graham Brown in 1880 con- 

 firmed this observation, and believed that they had excluded 

 the effect of the simultaneous dilatation of arterioles by the 

 fact that if reflex stopping of the heart was brought about, 

 so that the arterial pressure fell to zero, the dilated capillaries 

 did not empty. This, however, is not completely convincing, 

 because the elastic reaction of the capillaries might not be 

 sufficient to empty them after they had been stretched. If the 

 passive stretching had caused an elastic reaction, it is difficult 

 to see why the capillaries did not empty themselves back into 

 the arterioles where the pressure was zero. Better evidence of 

 independent action on the part of the capillaries is afforded 

 by the observation of these investigators that the diameter of 

 different capillaries is not in direct proportion to the arterial 

 pressure. Two capillaries lying side by side may require very 

 different external pressure to obliterate them, and, after a 

 pause, that one which previously collapsed under the lower 

 pressure may now require the higher one. 



In 1893, Worm-Muller had shown that large quantities of 

 blood could be injected into dogs without much rise of blood- 

 pressure. The blood remained in the circulation, and post- 

 mortem observations did not reveal a distension of the arteries 

 and veins of sufficient degree to accommodate it. The con- 

 clusion was drawn that it was accumulated in the capillaries 

 of the body generally. The adjustment is doubtless brought 

 about by a nervous reflex. It may be that a sufficient ex- 

 planation lies in dilatation of the arterioles and the resulting 

 passive distension of the capillaries. If this be not accepted, 

 and Krogh's results cast doubt on the acceptability of the 

 explanation, we must assume a direct nervous control of the 

 capillaries by vaso-dilator nerves. 



Severini described experiments on the empty capillaries 

 in excised tissues in which oxygen caused narrowing, carbon 

 dioxide widening ; but Roy and Graham Brown failed to con- 

 firm the observations. 



Other observations on capillary dilatation might be quoted, 



