Active Hypercemia. 



393 



dilator impulses similar to those found by Bayliss (1901) in the sensory nerves 

 of the limbs. 



5. Metabolites. 



Gaskell (1880) was the first to lay stress on the importance of the local 

 vaso-dilator action of acid metabolites ; he observed the effect of lactic acid on 

 the curarised mylo-hyoid of the frog, and measured with a micrometer eye- 

 piece the dilatation of the blood-vessels. About the same time Severini put 

 forward the view that " the increased flow of blood through an organ when it 

 is in a condition of activity is due to the trophic dilatation of the capillaries, 

 and not to relaxation of the vascular muscle." He stated that oxygen diminishes 

 the size of the capillary lumen, because the nucleus of the cells of the capillary 

 wall (nucleus of Golubew) becomes more spherical, while conversely with 

 the action of CO2 it flattens out in the cell, and so the lumen is greater. 



More recently, Krogh (1919) has observed the circulation in the thin 

 muscles of frogs and guinea-pigs, chiefly by reflected light, with a binocular 

 microscope ; he finds that when muscles contract, either spontaneously or as 

 a result of artificial stimulation, many more capillaries spring into view, and 

 when the contraction is over they disappear again ; capillary dilatation also 

 occurs when certain irritants and narcotics are applied locally to the tissue, 

 and in these cases the capillary dilatation is not to be explained by dilatation 

 of the arterioles. This independent state of contractility of the capillaries 

 is seen in the action of histamine, which dilates the capillaries and constricts 

 the arterioles (Dale and Eichards, 1918). This drug exerts such a powerful 

 action on the vessels that the injection into an animal of an amount equal to 

 one-millionth of its weight will cause a fall of blood-pressure to one-half 

 and a condition indistinguishable from surgical shock (Dale and Laidlaw, 

 1919). The possibility of the production in minute quantities of similar 

 substances during cellular activity must be kept in mind, though none have 

 been isolated up to the present. Still, it must be pointed out that the vaso- 

 dilation produced by the action of histamine in cats and dogs is replaced by 

 vaso-constriction in the guinea-pig, so that it would be totally unjustifiable to 

 assert that this substance plays the part of a general metabolite to bring 

 about vascular dilation in active organs. It must also be mentioned that 

 adrenalin in very small doses also possesses a dilator action on the capillaries 

 (an effect which appears to be independent of the sympathetic), but the 

 concurrent action on the arterioles is more pronounced than is the case with 

 histamine (Dale and Eichards, 1918). 



A similar condition of affairs is seen in chilblains, when the capillaries are 

 distended with blood of venous colour, owing to the impoverished oxygen 

 supply resulting from the simultaneous arteriolar constriction. 



