SUMMARY1. Four Merino wethers were exposed to dry bulb temperatures ranging from approximately 20 to 60°C, and the concurrent changes in respiratory frequency, tidal volume, respiratory minute volume, alveolar ventilation, dead space ventilation, carbon dioxide output, rectal temperature, and arterial and mixed venous blood, CO2 content, CO2 partial pressure and pH were established.2. The respiratory response to heat exposure showed two phases. Respiratory minute volume was initially increased by a rise in the respiratory frequency, while tidal volume decreased. After more prolonged exposure there was a second phase in which respiratory minute volume was further increased by an increase in the tidal volume; respiratory frequency was now slower than in the first phase but was still well above control values.3. The increase in respiratory minute volume during the first phase of the response was restricted almost entirely to the respiratory dead space; changes in blood CO2 and pH were slight. In the second phase, respiratory minute volume showed a much greater increase, and a change of alveolar ventilation to about 5 times the control level resulted in severe respiratory alkalosis.4. Contrary to findings in cattle, the slower, deeper form of respiration could be elicited even with rectal temperature in the normal range. This change in respiration appears to be the result of either peripheral thermoreceptor function or mechanical demands of the respiratory system. The neglect of control of acid-base balance during the second phase indicates the existence of a dominant thermal stimulus or modification of respiratory control mechanisms.

The values of hemoglobin concentration, Hb-O2 affinity and buffering capacity of the blood of six sea snake species considerably overlap values from terrestrial squamates. Decreased blood pH had little effect on the P50 but increased the n-value of Hb-O2 equilibrium curves. The O2 saturation of blood in the dorsal aorta varied between about 30 and 70% during voluntary diving in Acalyptophis peronii and Lapemis hardwickii. Voluntary dives ended when the lung PP02 was about 50 mm Hg and the arterial PO2 about 30 mm Hg indicating that roughly half of the O2 reserves had been used. In conjunction with relatively stable blood lactate concentration and pH, this indicates that voluntary dives occurred largely aerobically. In contrast, forced dives resulted in depletion of O2 reserves and large changes in blood acid-base balance. Long recovery periods following forced dives are inconsistent with field observations and thus suggest that extensive anaerobic metabolism does not normally occur in sea snakes. Bradycardia was not evident during forced dives. Large differences in PO2 between the lung and dorsal aorta indicated considerable right to left shunting either in the heart or in the lung. Venous blood represented over 50% of the systemic flow when there was considerable O2 in the lung. Therefore blood PO2 may remain relatively low despite elevated lung PO2 resulting from diving. In view of substantial capability for extra-pulmonary gas exchange, high shunting reduces the possibility of losing O2 through the skin and also may help prevent decompression sickness following deep dives.

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Summary Changes in the thickness of the spleen and associated alterations in the haemoglobin concentration in jugular venous blood were studied in undisturbed and excited conscious sheep. Decreases in spleen thickness were induced by single injections of adrenaline (1·4 to 35·5 μg/kg), and by disturbance. The increase in jugular vein haemoglobin concentration following splenic contraction returned to control levels after spleen thickness resumed its control value. However, the responses of these two variables differed markedly among sheep both to a given dose of adrenaline and to progressively higher adrenaline dosage. Respiratory events, feeding, changes in rumen pressure and in posture each induced characteristic variations in spleen thickness. No changes in spleen thickness were observed in conscious undisturbed sheep. Neither disturbance nor adrenaline injections altered the jugular haemoglobin concentration in splenectomized sheep.

ABSTRACT.-Heart rates, respiratory rates, body temperatures, ad libitum 'day'and 'night' food consumption and body weight changes have been examined in 15 mature Merino wethers shorn in moderate environmental conditions. All sheep showed a depression in food consumption for two days after shearing. Sheep that gained weight during the next three weeks then increased their food consumption at night by approximately 30% although the average daily consumption was only increased by 5%. Sheep that lost weight showed a depressed food consumption throughout the three week period after shearing. Marked increases in the temperature difference between ear skin and air as well as thermal tachypnoea during the warmest period of the day were recorded in all sheep 14-16 days after shearing. This indicated that the critical temperature for all sheep had decreased by about 10oc. These signs of acclimatisation appeared at similar times in all sheep, suggesting that increased resistance to body cooling developed at similar rates in weight gain and weight loss sheep and independent of the origin of body heat production. The results of the investigations are discussed in relation to the concept that the initial response to cold stress includes a depression in food intake and that the duration of this depression is a function of the cold stimulus and the strain it induces in the sheep.