THE NORMAL BEHAVIOR OF THE PULMONARY BLOOD VES- SELS WITH OBSERVATIONS ON THE INTERMITTENCE OF THE FLOW OF BLOOD IN THE ARTERIOLES AND CAPIL- LARIESr 2

In the work described in this paper, a method has been developed for direct microscopic observation of the smaller blood vessels of the mammalian lung in the unopened thorax. The behavior of these vessels and the flow of blood through them have been studied under a variety of conditions. We were particularly interested in their normal behavior, in the question of intermittence of the flow of blood in the arterioles and capillaries and t,he factors controlling it, and in the number of capillaries open at a given time and the variations in the circulatior within them. Observations upon the actions of various drugs on the pulmonary vessels are also presented. When this work was begun in 1924, Krogh (1919 and 1922) and Richards and Schmidt (1924) had reported the results of their brilliant work in which the Malpighian method of direct microscopic observation was used. The work of these investiga tors suggested the possi bility of the re-adaptation and modification of the Ma1 .pighian m .ethod for use in studying pulmonary blood vessels in the cat. Microscopic observations on the pulmonary circulation of the frog were first made and reported by Malpighi (1687). Stephen Hales (1733) and Tiemann and F. Roeder (1932) have also observed the flow of blood in the