Blood capillaries of the heart and other organs.

T HERE ARE, I believe, a good number of reasons for a renewed interest in problems of capillary permeability. To begin with, it is clear that we are dealing with a basic process in the physiology of metazoa. In these complex organisms, the life of the multitude of cells in the intimacy of tissue depends, in ultimate analysis, on the ample and continuous exchanges that take place across the wall of capillary vessels between the blood plasma on one side, and the interstitial fluid on the other. To continue, many pathologic conditions can be traced back to circulatory disturbances in general, and to variations in capillary permeability in particular, an outstanding example being the inflammatory process. And to finish, the mechanisms involved in the exchange of large quantities of water and solutes across the capillary wall are still poorly understood. Studies on the structural aspects of the problem, carried out over many years by light