Studies on the effects of osmotically active substances on the circulation and structure of the retina. I. Observations in vivo.

The retinal circulation in the cat teas studied by intravital microscopy, and experiments confirmed that the passage of red cells in the capillaries is intermittent, although plasma may be circulating continuously. Preferential pathways, in which blood fiows almost continuously, have been found in the superficial capillary network, forming a direct communication betioeen arterioles and venules. They form an alternative pathway for the blood tohich luoidd otherwise pass down to the deep plexus of capillaries. When distilled water is injected into the vitreous, the retina becomes edematous, usually causing narrowing of the arteries and veins and capillary closure. This closure may be due partially to the arterial constriction, but it teas thought that tissue tension and swelling of the capillary wall played a part in the effect. Hypertonic saline usually caused dilatation of the retinal vessels, but the effect was much more marked when it was used to reverse a previous closure induced by distilled water. Therefore, it seems possible that both tissue tension and swelling of the capillary toall (pericytes and endothelial cells) may be concerned ivith the autoregulation of the microcirculation in the eye. Furthermore, this mechanism of vessel closure might be involved in the development of some of the pathological lesions that characterize the retinopathies of diabetes mellitus, hypertension, and the collagenoses.