Microscopic Observations on Circulatory Systems of Living Transilluminated Mammalian Spleens and Parturient Uteri. ∗

A. Spleens.—The circulatory system of living spleens of mice, rats, and cats, were studied. The spleens were exposed through a small incision and vigilantly protected from minute temperature changes and trauma to prevent general vasomotor upsets in the organ. The spleens were transilluminated, using only the visible spectrum, by means of the previously described 1 quartz rod light. The linings of blood vessels, including sinuses, in living spleens, show as clear refractile borders, not to be confused with peripheral plasma layers of the blood. These linings are as continuous in living spleens as in other organs. Each vessel that I traced connected to both the arterial and the venous system; neither open ends nor blind ends of vessels were found. I saw blood pass through the red pulp via (1) long straight capillaries, (2) via the venous sinus systems and (3) via diapedesis, but I have not yet seen other types of passage. The distribution of blood to various areas and sub-areas of the red pulp is actively controlled by coordination of the action of powerful sphincter-like segments of branches of the arterial tree. Venous sinuses have a cycle of filling, storage and emptying. During filling the efferent end of the sinus is tightly contracted, whole blood flows into the sinus, plasma filters rapidly out of the sinus into the partitions which are usually termed pulp cords, leaving the sinus distended up to 20 to 50 times its original diameter with solidly packed blood cells. The retention of blood cells lasts from a few minutes to several hours, At emptying, the efferent end of the sinus relaxes suddenly, the packed blood cells emerge in masses, and the sinus decreases in diameter, quickly, until it is but 2 or 3 times the diameter of a red blood cell and then it conducts blood like any other blood vessel.