142 HISTOLOGY 



Other animals are able to produce a considerable amount of heat by 

 their activity. A mackerel is said to raise its body temperature eight 

 degrees above that of the water by its vigorous swimming. In rest, how- 

 ever, it returns almost to the temperature of the water. 



Among one group of animals only is the function found of maintain- 

 ing a temperature constantly above and independent of that of their 

 surroundings. These are the vertebrate animals, and only two divisions 

 of these, the birds and the mammals, do this. In man the temperature 

 is about thirty-eight degrees, while in some birds it is constantly as high 

 as forty-four degrees. Not only are these temperatures high, but they are 

 constant within very small limits. To secure this constancy there must 

 be means of producing more heat when it is too low, of lowering it 

 should it get too high, and of properly distributing it as well as retaining 

 it in the body against radiation. 



Heat-production must be stimulated by its need, by nerves which, 

 when the temperature gets too low, automatically cause a greater produc- 

 tion and oxidization of thermochondria. Also muscular exercise liber- 

 ates much heat due to the myochondria in their work of heating the 

 myo-fibrils to make them absorb water and contract. 



This heat is distributed from its points of generation by radiation and 

 by the circulation of the blood. When the body becomes too warm, 

 heat is removed by the evaporation of fluids on the body surfaces. Sweat, 

 on the outer body surface in the horse and man, and other fluids in the 

 throats of animals like the dog and the common domestic fowl, which 

 "pant" when too warm, are the fluids used. The extremities of these 

 animals, as well as the surfaces, are sometimes much below the body 

 mass in temperature. 



There is but little histology to exhibit concerning this function, 

 although there are tissues more or less set aside to perform it. As 

 mentioned before, the muscles probably produce most of the heat, the 

 blood distributes it, and the surfaces of the body release it and lower the 

 temperature. Any accident or pathological condition in these may 

 cause irregularity in the temperature, sometimes to the point of killing 

 the organism by a reduced or excessive amount of heat. 



LITERATURE 



VERWORN, M. " General Physiology." 

 FOSTER, M. " Physiology." 



