As a part of serial studies on the peripheral blood vessel responses to physical exercise, an observation was made on the changes in retinal blood vessel diameters due to physical exercise, as they are presumed to reflect the changes in the cerebral circulation. Photographs were taken with an non-mydriatic ophthalmo-fundo-camera in 8 phases at rest, during and after exercise. Regarding the diameters of retinal vessels, those of A. and V. temporalis superior and inferior were measured at the papillar brim and the values were expressed in the ratios to the papillar diameter. The hearf rate and the arterial blood pressure of the upper arm were also measured simultaneously with the photography. It was disclosed that the diameters of all the retinal blood vessels were enlarged due to exercise, and reached the maximum in 20 minutes of exercise, amounting to increase of 13-21% on the average. However, they showed decreasing tendencies thereafter even though the exercise continued. In 20 minutes after cessation of exercise, they recovered the initial resting values. Young men and elder ages showed similar tendency, but the increase in diameter was greater in young men than in elder men. If the assumption, that the changes in the cerebral circulation are reflected in the retinal circulation, can be accepted, the increase in diameters of the retinal vessels seems to suggest the augmentation of cerebral circulation during the physical exercise. The regulation of blood flow in the peripheral vessels is considered to play an important role during exercise in transport of substrates as energy source and their metabolites (Tsutsumi et al., Shibayama and Ebashi), because the metabolism is augmented in active organs (A^^°strand and Rodahl, Ohira et al,. ). The present study was made with an intention to observe the changes in blood flow in retinal vessels during exercise. The retinal arteries are the peripheral branches of the internal carotid artery (Henry and Meehan), and, accordingly, it seemed to the authors that the changes in the intracranial blood flow during exercise might be reflected on the changes in the diameters of retinal vessels.
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