ANIMAL HEAT. 487 



radiation and conduction is facilitated. The influences of external tempera- 

 ture are even more potent in their effects than those of internal temperature, 

 chiefly because of the much wider range of temperature to which the organism 

 is subjected. Bodily temperature under ordinary circumstances does not vary- 

 more than 1° to 2° C. during the twenty-four hours, but external temperature 

 may vary as much as 40° C, or more. External heat tends by exciting cuta- 

 neous nerves to reflexly diminish heat-prod notion and thus indirectly dimin- 

 ish heat-dissipation ; but this is to some extent antagonized by a dilatation of 

 the blood-vessels of the skin, an excitation of respiration, and increase in the 

 quantity of sweat, all of which tend to increase heat-dissipation, but which 

 are unable to balance the opposite effects. Cold, on the other hand, accelerates 

 both heat-dissipation and heat-production. The loss of heat from the body 

 is increased because of the greater difference in the temperatures of the body 

 and the surroundings ; but, on the other hand, the cutaneous vessels are con- 

 tracted, the circulation is less active, and the quantity of sweat is lessened, all 

 of which are unfavorable to heat-dissipation. Yet while these latter altera- 

 tions tend to diminish heat-loss, they are not sufficient to compensate for the 

 increased expenditure by radiation and for the greater loss by respiration. 



Circumstances which increase heat-production above the normal tend indi- 

 rectly to increase heat-dissipation. Other things being equal, the greater the 

 quantity of heat produced the greater the heat-dissipation, unless the bodily 

 temperature be below the normal, in which case heat-production may be in- 

 creased and yet heat-dissipation remain unaffected, or even be diminished, until 

 sufficient heat has accumulated to bring the bodily temperature up to the 

 mean standard. 



The larger the surface of the body exposed to the normally cooler sur- 

 roundings, the greater is the loss of heat. The larger the animal the greater the 

 body-surface, and therefore the greater is heat-dissipation ; but in proportion 

 to body-weight smaller animals have larger body-surfaces, therefore heat-dissi- 

 pation is relatively greater, although not absolutely so (see p. 4.°><M. The area 

 of body-surface involved in heat-dissipation is affected by the position of the 

 individual. Thus, by bringing the arms and legs in contact with the body 

 the total surface exposed is lessened. On the other hand, animals which 

 habitually have their legs in apposition with the trunk have their radiating 

 surfaces increased when their legs are extended. For instance, in the rabbif 

 extension of the legs enormously increases heat-dissipation, so that the bodily 

 temperature is profoundly affected. 



The condition of the vascular system exercises an important influence. 

 Circumstances that excite the circulation affect heat-dissipation both directly 

 and indirectly. Thus, heat-loss is directly increased by the excitatioD of the 

 respiratory movements, by the increased secretion of sweat, and by the larger 

 supply and increased temperature of the blood to the skin. Increased activity 

 of the circulation also increases heat-production, ami thus indirectly affects heat- 

 dissipation. Opposite conditions, of course, lessen heat-dissipation. 



The larger the quantity of air respired, other things being equal, the larger 



