594 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



quality. Woollen goods compared with those made of cotton not only have 

 a far greater absorptive capacity but retain moisture for a longer time. When 

 the clothing is of wool people are less apt to catch cold from exposure to 

 draughts and sudden cold than when it is of linen or cotton, the wool pre- 

 venting a too rapid evaporation of moisture, thus guarding against chilling. 

 Porosity is a comparatively subsidiary factor. The greater the weight of the 

 clothing, other things being equal, the more is heat-dissipation hindered. The 

 color of the outer apparel has a certain influence owing to the relative heat- 

 absorbing capacities, black clothing being warmer than white, etc., hence the 

 general use of white or light-colored clothing in warm climates and seasons. 



A rise of internal temperature (bodily temperature) is favorable to an in- 

 crease of heat-dissipation, for several reasons : (1) Heat-production tends to 

 be increased and thus cause an effort of the system to get rid of the excess of 

 heat. (2) The activity of the circulation is increased, causing a larger amount 

 of blood to be brought to the cutaneous surface where it is subjected to the 

 influence of the cooler surroundings. (3) Respiratory movements are increased 

 so that heat-dissipation is favored by the larger amount of air respired and 

 larger amount of moisture carried off. (4) The temperature of the body is 

 higher in relation to that of the surroundings and thus heat-dissipation by 

 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-production 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- 



