Cerebral Circulation

In the earlier papers of this series evidence has been presented to show that the caliber of arteries supplying the mammalian cortex is regulated by at least three mechanisms: first, by sudden and extreme alterations in systemic arterial pressure;¹second, by vasomotor nerve impulses transmitted to arteries and arterioles of the pia;1athird, and much the most frequently active mechanism, as well as the strongest, by chemical influences acting directly on the blood vessel walls (either on the muscle fibers themselves or through some local reflex). During investigation of the last of these mechanisms it was found that various chemical substances, histamine,²acetylcholine,³amyl nitrite³and lactic and hydrochloric acids,⁴and increased tension of carbon dioxide⁵caused dilatation of blood vessels in the pia. The present paper adds to this list another acid—hydrogen sulphide. This gas diffuses rapidly through living membranes, notably the