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H. N. Upadhyay.
Significance of Regulation of Temperature in Mammals
It is difficult to overestimate the importance of the advance in living organization that is made possible by the maintenance of a high and constant temperature. Sir Joseph Barcroft pointed out that many refinements of organisation can only operate under constant conditions. For instance, if there is a constant temperature, elaborate patterns of activity can be set up, in the cerebral Cortex, allowing for persistent and complicated memories. Similarly, in various parts of the body there are intricate sets of Biochemical reactions, that would be disturbed by large temperature fluctuations. At the same time achievement of high temperature allows a greatly increased level of activity. The birds and Mammals have been experimenting independently with high temperature for probably more than 100 million years, but it may be that one or both groups will eventually make still more spectacular innovations of organisation on this basis, including perhaps the use of still higher temperature.
It is not known how temp-regulating mechanism first arose. The Prototherians (Egg laying mammals).
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Platypus and Echidna posses hair and they probably diverged from the other mammals, not later than the early Jurassic period, nearly 150 million years ago. Therefore it seems that the mammalian like began to be warm-blooded earlier than this date, as also did the line that was leading to the birds. This may have been a response to either cold or warm conditions; reptiles are severely limited in their distribution by temperature. It is also possible that the condition did not follow any special climatic change but that the early Avian and Mammalian Stocks were pioneers, driven by the competition of their many reptilian cousins to seek life in colder or hotter land regions, which were not yet inhabited by tetrapods,
Cold will make a reptile dormant, unless the animal can be active enough to keep itself warm by the heat produced as a by-product of muscular activity.
This will be made more easy, if the animal is large and, of couse, especially if a heat insulating mechanism is developed. It is not difficult to understand how a temperature above that of surroundings could be achieved by sufficiently active reptilian animals.
Even at the present day the heat of muscular work, remains the chief source of heat in Mammals. In the early stages of the evolution of high temperature, alteration of heat production, was probably the main means of temperature regulation, as it still is today in monotrems and bats. In all mammals a fall of external temperature calls forth extra muscular activity by shivering. The high mammals also possess a mechanism for the control of heat loss and they maintain a constant temperature largely by this downward regulation.
Control of Temperature-regulating mechanism is centred on the Hypothalamic region of the forebrain, especially in the Tubercinerum, which is large in birds and mammals. In this region their are cells, that serve as detectors; when the blood is too hot or too cold, nerve impulses are sent out to vary the rate of heat production or heat loss, and the temperature is kept steady with little oscillation. After removal of the tuber an animal no longer regulates properly, its temperature fluctuates with that of its surroundings. Other parts of the Brain also play a large part in regulation and it has long been known that stimulation of the Caudate nucleus of Corpus striatum, for instance by puncture, causes a temporary rise in the
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