THE CONTROL OP THE CIRCULATION 253 



because the oxygen is so rapidly used up. In lirine, where this is not the 

 case, the pressure of 2 is about the same as in the venous blood, and when 

 a neutral gas is placed in the pleural or peritoneal cavities, it ultimately 

 becomes mixed with oxygen up to 3-4 per cent. (Tobiesen cf. Krogh.) 



Still more definite proofs of the ability of capillaries to dilate inde- 

 pendently of changes in the corresponding arterioles have been obtained 

 by Krogh and by Hooker. co Krogh found that irritation of the ventral 

 aspect of the frog's tongue, as by punctate stimulation with a hair, caused, 

 after a brief latent period, a marked vasodilatation and increased blood- 

 flow. This dilatation was seen by the microscope sometimes to affect both 

 the capillaries and small arterioles over a considerable area. That the 

 former had been actively dilated, and not merely passively distended by 

 greater inflow of blood from the arterioles, was shown by the fact that 

 sometimes it was possible to restrict the irritation so that only a capillary, 

 or even a short piece of one, dilated. In such cases the dilatation often 

 started at the venous end of the capillary and spread towards the arterial 

 end. This made it appear as if the blood were flowing towards the heart. 

 After clamping the lingual artery so as to slow down the circulation, irri- 

 tation caused the capillaries to dilate without any increased flow of blood 

 through them, this occurring, however, immediately the clamp was re- 

 moved. Urethane or epinephrine, applied locally, dilated the capillaries, 

 but not the arterioles. 



After cocainization or degeneration of the nerves, the dilatation in- 

 stead of being diffuse was then more localized; merely cutting the 

 nerves, however, had no effect. This indicates that a local nerve axon 

 reflex, similar to that described on page 898, is concerned in the spread. 

 There must be sensory nerve fibers on the walls of the smallest vessels 

 and the impulses which are set up by their stimulation must travel up 

 the nerve to a collateral, down which they then pass to neighboring 

 parts of the vessel as dilator impulses. Hooker has also shown that 

 stimulation of the cervical sympathetic in the cat causes the capillaries 

 around the hair follicles of the ear to exhibit movements which are inde- 

 pendent of changes in the arterioles. These discoveries explain the 

 association of local hyperemia with cyanosis (the blood is flow- 

 ing so slowly in dilated capillaries that its oxygen is used up), 

 the pathogenesis of shock (page 307), the initial stages of inflamma- 

 tion, and it is altogether likely that they will before long lead us to an 

 explanation of the causes for clinical hypertension. It is possible that 

 examination of the capillary circulation in the skin at the base of the 

 finger nails by Lombard's method 62 * would be of considerable value in 

 the differentiation of cases of hypertension. 



*A drop of glycerine is placed on the skin at the base of the nail, with the hand firmly 

 supported on a rest, and a strong light thrown on it. An objective magnifying up to 35 times 

 is then focussed until the capillary loops are seen. 



