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1 The Department of Physiology, Albert Einstein College of Medicine, New York, New York 10461
2 The Marine Biological Laboratory, Woods Hole, Massachusetts 02543
Very large pressures of oxygen are found in the vitreous humor close to the retina of many teleosts and of the holostean, Amia. Elevated pressures are not found in species lacking a choroid rete mirabile. In species having choroid retia, the observed oxygen pressure roughly parallels the extent to which the rete is developed. On this evidence we suggest that the choroid rete mirabile, acting in concert with the pigment cell layer of the retina, plays an essential part in establishing the large oxygen pressure at the retina.
The choroid rete mirabile of the holostean, Amia calva, is essentially similar to the teleost choroid rete, from which it differs only in details of structure.
The capillaries of the counter-current organ, the choroid rete, are short, compared to the capillaries of the swimbladder rete, and are very nearly the same length in all the fishes examined, independent of the size of the fish, which ranged from tens of grams to hundreds of kilograms. The increased demand accompanying larger size is met by increasing the number of capillaries. We conclude that the structure of the choroid rete mirabile is adapted to supply a large flux of oxygen toward the retina at a relatively low pressure.
The general features of the structure of the choroid rete mirabile are largely similar to those of the swimbladder rete mirabile, but the two retia differ sufficiently to suggest that the physico-chemical mechanisms by which they build up large oxygen pressures may also differ in detail.
It is suggested that movements of water between inflowing and outflowing capillaries of the rete mirabile may play an important role in the mechanism by which oxygen is secreted into the eye and into the swimbladder.
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