The testicular artery in mammals convolutes to a variable extent before reaching the testis, and is closely surrounded by the veins of the pampiniform plexus. Evidence is presented suggesting that this vascular mechanism has a thermoregulatory function for the testis.

Species differences in the degree of convolution and calibre of the testicular artery and its relation to the veins of the pampiniform plexus would appear to account in some measure for observed differences in abdomino-testicular temperature gradient.

Experiments on dogs, rams, and goats also indicate that the close relation of the pampiniform plexus of veins to the testicular artery is well adapted to bringing about preheating or precooling of arterial blood flow to the testis.

We are indebted to Prof. H. C. Bazett for the method of soldering the thermocouples and gifts of plastic tubing, and for his interest in the initial stages of these experiments. We also wish to thank the late Dr A. E. Barclay for his help with the radiography in this investigation.

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