Responses of unshorn and shorn sheep to thermal stress.

1. The effect of close shearing upon the respiratory frequency of ewes at ambient temperature of 20 degrees C and 55% relative humidity has been described.2. Removal of an adequate area of fleece (3000 cm(2) or more) on the thoraco-lumbar region resulted in a depression of respiratory responses and this was attributed to the exposure of cold receptors located beneath the fleece.3. The continued depression of respiratory frequency of partially shorn sheep (3000 cm(2)) during rising ambient temperature appears to be due to persistence of cold receptor drive originating from the shorn area.4. The absence of any rise in carotid blood temperature before the onset of polypnoea in unshorn sheep, during rising ambient temperature, suggests that the onset of panting in sheep can occur almost immediately upon a rise in ambient temperature even in the absence of any rise in the temperature of the blood supplying the brain.

Role of naso-buccal passages in thermoregulation in sheep.

1. Experiments with a face-mask in which the temperature of the air in the face-mask was raised to 40 degrees C while the ambient temperature in the chamber was maintained at 20 degrees C, resulted in a marked increase in respiratory frequency and a slight decline in carotid blood temperature of unshorn sheep. Partially shorn sheep showed only small respiratory responses.2. Localized infra-red irradiation of the naso-buccal area of unshorn sheep also resulted in an increased respiratory rate.3. It is suggested that the initiation of polypnoea during infra-red irradiation of the naso-buccal region and following rise in the temperature of the air in the face-mask is due to stimulation of warm receptors in the upper respiratory tract.4. Cooling the naso-buccal air in the face-mask to 10 degrees C after thermal polypnoea had been established at an ambient temperature of 40 degrees C resulted in a moderate decline of 30-40 respirations/min. This decline was attributed to the stimulation of cold receptors located in the upper respiratory tract.

Reflex onset of polypnoea in sheep.

1. Measurements have been made of the cardio-respiratory activities, ear and lumbar skin temperatures, and temperature of the blood in the carotid artery and jugular vein of partially shorn and unshorn sheep during mammary heating at ambient temperatures of 20/55, 15/55 and 10/55 (dry bulb temperature, degrees C/relative humidity,%).2. Heating the mammary region resulted in a marked rise in respiratory and heart rates of unshorn sheep, whereas, in partially shorn sheep, the heart rates increased without being accompanied by a similar rise in respiratory rates.3. With decrease in ambient temperature from 20 to 10 degrees C, there was a decrease in respiratory response of unshorn sheep during mammary heat treatment suggesting that the magnitude of respiratory response during mammary heat treatment depends on the environmental temperature to which the sheep is exposed.4. The carotid blood temperature declined following mammary heat treatment at all ambient temperatures studied, both in unshorn and partially shorn sheep, but the magnitude of decline was greater in unshorn than in partially shorn sheep.5. The rise in respiratory rates following mammary heating of unshorn sheep at all ambient temperatures, in spite of a decline in carotid blood temperature and the absence of similar responses during flank heating, is attributed to the stimulation of warm receptors in the mammary region.

Thermal response of intravascular and rectal tissue to temperature changes and chemical conditions in the rumen of sheep.

1. Experiments were conducted with two wether sheep which were fitted with rumen cannulae and chronically implanted intravascular thermocouples. An attempt was made to study the thermal response of intravascular and rectal tissue to temperature changes and chemical conditions in the rumen.2. When ice or hot water were placed in the rumen there was an immediate fall or rise in the intravascular temperature accompanied by a similar change in rectal temperature. The intravascular temperatures returned to their precooling or preheating level of 40 degrees C within 130 min, the rectal temperatures required 6-8 hr to return to their pretreatment values.3. When 0.5 M acetic acid was infused into the rumen there was a marked rise in the intravascular temperature, over and above the diurnal rhythm but not in rectal temperatures. Infusion of mixtures of acetic plus propionic or acetic plus n-butyric acids caused an intravascular temperature rise on feeding within the range of the diurnal pattern. In fasted animals, infusions of volatile fatty acids resulted in no rise in intravascular temperature.