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1. Oxygen consumption, respiratory frequency, and temperatures of the rectum, common carotid artery, external auditory meatus, and skin on the ears, near the distal end of the metacarpus and metatarsus, upper thigh and mid-side of the body, were measured in five rams before, during and after heating the scrotum. Effects on the woolly animal exposed to ambient temperatures of 17-19° C, and on the shorn animal exposed to ambient temperatures of 5·6-24·5° C, were determined.

2. In eight experiments on woolly animals scrotal heating resulted in vigorous panting and a lowering of deep body temperatures by 0·3-1·1° C within 50 min. There was no significant change in oxygen consumption or body skin and upper thigh temperature. Changes in skin temperature of the extremities indicated, on average, a vasodilator response to heating. However, individual responses were unpredictable, there sometimes being dilation and constriction in different extremities of the same animal.

3. In twelve experiments on shorn animals in environments below 20° C, oxygen consumption before scrotal heating was higher than when the animal was woolly. Scrotal heating resulted in a fall in oxygen consumption and a lowering of deep body temperatures by 0·2-0·9° C within 50 min. There was no change in respiratory frequency or body skin and upper thigh temperature. As in the woolly animal, vasomotor responses were disorganized, although there was a tendency towards vasodilation.

4. In three shorn animals in environments of 20·5 and 24·5° C, oxygen consumption before scrotal heating was only slightly higher than when the animal was woolly. In two of these animals (in 20·5 and 24·5° C) scrotal heating resulted in a slight lowering of oxygen consumption and mild panting, and deep body temperature fell by 0·9° C in 50 min. In the third animal (in 24·5° C) there was no change in oxygen consumption but vigorous panting and vasodilation, and deep body temperature fell by 1·3° C in 50 min.

5. In a number of experiments on shorn animals scrotal heating was continued for more than 100 min. Deep body temperature was controlled at a new low level after the initial fall, and the possibility of a resetting of the `set-point' temperature is discussed.

6. The previously unexplained fall in body temperature of the shorn ram in which panting did not follow scrotal heating may now be mainly ascribed to a reduction in metabolic rate. Also, the possible role of cutaneous evaporation should not be discounted. The inhibition of panting in the shorn sheep at environmental temperatures below 20° C remains unexplained.

7. Body heating alone, by covering the shorn animal with a sheep skin coat in an environment of 19° C, elicited well organized vasomotor changes in the ears and lower legs.

8. As a result of the present study and previous work by others, it is clear that all thermoregulatory mechanisms, with the exception of vasomotor changes, may be influenced in a predictable manner by temperature changes of the scrotum.

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