THE CONTROL OP THE CIRCULATION 251 



animal is now placed in the vertical tail-down position, however, it falls 

 to the zero line and the animal soon dies (Fig. 78). The influence of the 

 third factor is not so great as of the other two, but can be shown by the 

 increased respiratory activity which is likely to develop in the vertical 

 tail-down position, the anemic condition of the respiratory center being 

 no doubt the cause of the increased respiration. 



THE CAPILLARY CIRCULATION 



It has been the custom to assume that the walls of the capillaries are 

 incapable of constricting or dilating independently of changes of pressure 

 in the blood circulating in them. According to this view the magnitude 

 of the capillary circulation, the pumping action of the heart being constant, 

 depends primarily on the state of contraction or dilatation of the arterioles 

 from which they spring and secondarily, on the venous pressure; when 

 the arterioles are dilated, the pressure will rise in the capillaries, causing 

 them to become passively dilated, and when the arterioles are constricted, 

 the capillaries in virtue of their elasticity will contract again. 



Krogh 57 has recently brought forward unassailable evidence to show that 

 this conception is wrong. He has shown not only that the capillaries 

 possess powers of constricting and dilating quite independently of the 

 arterioles, but also that their caliber when the tissue they supply is at 

 rest is very much less than when the tissue is active, indicating therefore 

 that they exist in a condition of constrictor tone. These discoveries were 

 made by examining, chiefly by reflected light, with the binocular 

 microscope thin muscles in living frogs and guinea pigs (under urethane), 

 or by injecting intravenously a solution of india ink, then killing the 

 animals, and examining either fresh or fixed tissues by the microscope to 

 determine into which capillaries the black particles of the ink had pene- 

 trated. In resting muscles it was found that relatively few capillaries are 

 visible, these being however evenly distributed, and forming an elongated 

 mesh-work along the fibers. When the muscles contract (either spontane- 

 ously, or as a result of artificial stimulation) many more capillaries spring 

 into view, and when the contraction is over they disappear again. This 

 microscopic evidence of extreme variability in open capillaries was con- 

 firmed by noting the color of the muscles after injections of india ink; 

 those that had been resting before the animal was killed being stained only 

 a faint grey, whereas those that had been active were almost black. 



Another fact of very great importance, which was revealed by these in- 

 vestigations, was that the blood corpuscles often crowd themselves through 

 capillaries having diameters that are much less than those of the corpus- 

 cles. The average diameter of the capillaries in the resting muscles of 

 fm<rs is 4.5 /*, whereas that of the corpuscles is 22 //, (long) and 15 n 



