Acclimatization and ventilation studies of cattle: with special reference to the histology of cow skin, the main organ of heat dissipation
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1. Twenty -one different regions of the skins of five 3 to 4 year old Ayrshire cows, one 9 year old Ayrshire cow and an 8 day old Ayrshire calf have been examined histolo; ,ically in an attempt to determine the distribution and structure of the 'sweat' glands and their mode of secretion. The capillary supply in the skin and of the sebaceous and 'sweat' glands was also studied.2. The number, length and circumference of the 'sweat' glands and the area of their secreting surface were determined and the results statistically analysed. The diameters of the capillaries, the circumference of the capillary loops and the surface area of the capillaries of the capillary network at the sub - epidermal level were also measured and analysed.3. It was ewtablished that in all the body regions each hair follicle was invariably associated with a 'sweat' gland and also with a group of sebaceous glands and an arrector pill muscle. This inseparable combination was named a 'hair follicle unit'.4. The number of 'sweat' glands per unit area of skin was much greater in the calf than in the caws.5. The secreting surface of the 'sweat' glands of the calf was found to be larger than that of the cow per unit area of skin, but in this respect no significant difference was found between the five 3 to 4 year old cows and the 9 year old cow.6. The area of the secreting surface of the 'sweat' glands per unit area of skin was least in the sacral, forehead, lower hind -leg and udder regions. Other regions differed little from each other in this respect.7. The area of the capillary surface in the rete sub -epidermius per unit area of skin was greatest in the ventral region of the neck and in the upper foreleg and least in the sacral and lower fore and hind -leg regions.8. The 'sweat' glands were found to be apocrine, and the sebaceous glands holocrine.9. A capillary network was observed to be present in the sub -epidermal level of the corium in all the body regions. It was named the rete sub -epidermius.10. Each hair follicle was encircled by one capillary loop of the rete sub -epidermius.11. The area of the capillary surface of the rete sub -epidermius per unit area of skin was less for the 9 year old cow than for the 3 to 4 year old cows.12. The sebaceous glands were well supplied with capillaries, but the capillary supply of the 'sweat' glands was so poor that it seemed unlikely that they could obtain sufficient fluid to make their secretion of any importance in the dissipation of heat by evaporation from the skin surface.13. The contracting arrector pili muscle in addition to its normal function may aise) exert pressure on the rete sub-epidermius and thus reduce heat loss by retarding blood flow in the capillaries.