466 Journal of Agriculiiirc. [8 August, 1907. 



reduced. An instrument on the same principle, but far more delicate, 

 is the thermostat of the chemist which can be kept constant to within a 

 one fiftieth of a degree Fahrenheit. Here too the flame yielding the 

 heat is reduced when the temperature of the thermostat rises only a small 

 fraction of a degree. But the incubator or thermostat must be kept in a 

 sheltered room and cannot be exposed to rain and snow and frost ; more- 

 over the combustion in the flame cannot be alloiwed to fluctuate within 

 wide limits. The superiority of the homoiothermal animal to such a 

 mechanism lies in the fact that not only can it regulate its heat supply 

 but also can regulate its heat loss. 



How is the heat supply regulated ? When a mammal or bird is exposed 

 to atmospheric conditions that check heat loss and therefore tend to allow 

 the heat of the body to accumulate we find that, through the agency of 

 the nervous system, the muscle tone is reduced and that the animal instinc- 

 tively avoids strenuous exertion. There is less combustion in the body as 

 can be shown by the diminished consumption of food and the decreased 

 output of carbon dioxide. Conversely when the animal is exposed to 

 cold its muscle-tone is reflexlv raised, it instinctively takes exercise, and 

 its consumption of food and output of carbon dioxide may be more than 

 doubled. That play of muscles which we call shivering is a muscular effort 

 for the sole purpose of raising the temperature. A bat waking from its 

 winter sleep resorts to shivering to Avork up its temperature rapidly and 

 animals in fever show the same thing. 



How is the heat loss regulated? An animal mav be called upon to 

 do strenuous work irrespective of weather conditions ; moreover there is 

 in the resting animal sufficient combustion going on to make the temperature 

 mount up if the heat loss from the skin is seriously interfered with. We 

 shall find in the homoiothermal animal that there are continual adjustments 

 of the heat loss to suit the altered combustion within and the altered state 

 of the air without. In the first place, if, through anv reason, it is neces- 

 sary to check heat loss the blood vessels in the lower layer of the skin 

 contract, so that less blood flows through the skin and less heat is lost 

 by radiation and conduction. We also find in most Inrds and mammals 

 that the little muscles attached to the roots of the hairs or feathers, con- 

 tract so that the external coat is ruffled and made therefore a poor con 

 ductor of heat. Man possesses these muscles, but their contraction in cold, 

 producing goose-skin, is useless as he has lost in the process of evolution his 

 hairy covering. Many animals reduce the surface of the exposed skin 

 by cowering or huddling the limbs together. 



If however the heat of the body threatens to accumulate then various 

 devices can be brought into play to facilitate heat loss. The superficial 

 vessels can be enlarged and the skin flooded with quick flowing blood so 

 that radiation and conduction are increased. In man and the horse sweat 

 can be poured out in great volume over the skin. Further, the breathing 

 may be increased so that loss of heat from the lungs and air passages and 

 even mouth may be increased ; this reaction is more marked in the non- 

 sweating animals, witness the lolling tongue and panting respiration of a 

 dog after vio'lent exertion or even when artifically heated. 



The exact mechanism by which heat is regulated is not clearly under- 

 stood. There is undoubtedly in the central nervous system and most 

 probably in the medulla oblongata a nerve centre exquisitely sensitive 

 to changes in temperature and from which nerve impulses can pass to the 

 skin, blood vessels, muscles, etc. 



