962 NUTRITION AND HEAT REGULATION. 



known to everyone that they are made into a great variety of 

 materials, each possessing certain pecuhar physical properties. 

 Speaking in very general terms we may distinguish between the 

 woven materials in which there is a warp and a woof, and the 

 knitted materials in which the thread or yarn is looped or knotted. 

 By the latter form of manufacture the porosity of the fabric, that 

 is, the amount of air entrapped in the meshes, may be greatly 

 increased, and thus its property as a non-conductor of heat may 

 be improved. The character of the clothing to be worn depends 

 naturally upon whether it is intended to diminish the loss of heat 

 from the body, as in cold weather, or to facilitate this loss, as in 

 warm weather. In the latter case it should be sufficiently loose, 

 porous, and permeable to permit easy evaporation of sweat and 

 free circulation between the shirt-skin layer of air and the outside. 

 The knitted cotton goods seem to fulfil these conditions better 

 than the closely woven goods, especially as underclothing for those 

 engaged in muscular work. The smooth-woven cotton easily be- 

 comes saturated with sweat and then forms practically a sheet of 

 water which retards evaporation from the skin, and increases the 

 danger of injurious local chilhng from drafts. In very hot weather 

 and in tropical climates there is a further advantage in having the 

 outer garments light rather than dark in color, since when exposed 

 to the sun the black fabrics absorb much more heat than the white. 



Physical Regulation of Heat Loss. — The most important means 

 of controlling the heat loss, however, is by automatic reflex con- 

 trol through the sweat nerves and the vasomotor nerves. By 

 these means the amount of perspiration evaporated from the skin 

 and the amount of warm blood sent through the skin are con- 

 trolled. Rubner speaks of this side of the heat regulation as the 

 physical regulation. By its means the body may be safe-guarded 

 from an abnormal rise of temperature. In warm weather the 

 secretion of sweat is greatly increased by reflex stimulation of the 

 sweat nerves. The greater amount of water requires a greater 

 amount of heat to vaporize it, and thus the heat loss is increased. 

 The value of this control is illustrated by a case recorded by Zuntz* 

 of a man who possessed no sweat glands. In summer this indi- 

 vidual was incapacitated for work, since even a small degree of 

 muscular activitv would cause an increase in his body tempera- 

 ture to 40° or 41° C. 



The control through the vasomotor nerves is also important. 

 The blood-vessels bring the warm blood to the skin, where it loses 

 its heat by conduction and especially by radiation to the cooler 

 air. When the surrounding air is much below the temperature of 

 the body the vasoconstrictor center is stimulated, the blood- 

 * Zuntz, "Deutsch. medizinal-Zeitung," 1903, No. 25. 



