238 NUTRITION 



Voluntarily the loss of heat is decreased by man by wearing more 

 clothing or furs, which besides being themselves non-conductors of heat, 

 help to keep a layer of "dead," non-conducting air about the body. 

 In the lower animals this fact is obvious, many animals having two suts 

 of fur or of feathers of very different degrees of "warmth." 



When the body-heat becomes or tends to become abnormally high, 

 in general the opposite physiological movements occur. The dermal 

 blood-vessels under the influence of the vaso-motor centers dilate, and 

 much more' blood being then forced into the skin, radiation and conduc- 

 tion increase and sweat is more freely poured out. The latter not only 

 aids thermolysis by evaporation, but it makes conduction and radiation 

 more active by increasing the conductivity of the skin. It acts, also, 

 and more importantly, by pouring the water to be evaporated outside 

 the oily and ill-conducting epidermis, it being the sebum and not the sweat 

 which contains fat. This dermal vaso-dilatation is probably the most 

 active means of heat-regulation, and it serves as a prompt and vigorous 

 agent in expending surplus heat. Because of its failure in fever, owing 

 to some disturbance in the medullary centers, the temperature rises ; this 

 increases metabolism, which in turn raises the heat still more. Many 

 other influences at different times act in a similar way, chief among these 

 being bacterial irritations from toxins. Sometimes during fever sweat 

 is secreted, but it is exuded on a cool surface ("cold sweat") and usually 

 under conditions which largely prevent its proper antipyretic effect. 'It 

 is sweat-secretion of this purely "nervous origin," unaccompanied by 

 dermal vaso-dilatation, which comes from some emotions, especially terror. 



With the reflex vaso-dilatation in the skin may go the other more 

 voluntary conditions, already noted, useful for the reduction of body- 

 heat, such as frequent bathing and the removal of clothing. Any 

 circumstance, in short, which will help to expose to a cooler environment 

 (air or water) a larger amount of body-heat than before serves to cool 

 the body, heat-production being at the same time reduced reflexly, 

 instinctively, or voluntarily on a basis of instinct or of comfort. 



THE THERMOTACTIC NERVES. The neural mechanism of heat- 

 regulation is as yet in its details not very well known. There is good 

 functionary evidence that there is something like a reflex arc for this 

 thermotactic purpose. The tissues have means of sending information 

 concerning their thermic condition to the brain, which thereupon sets 

 in motion the regulating mechanism in the direction required, either 

 to stimulate the tissues (especially the muscles) to produce more heat or 

 to stimulate the skin to lose more heat, or vice versa. As will be seen 

 more fully in the chapter on the Sense Organs, there are minute spots 

 scattered through the skin, some of which respond to stimulation by 

 heat and some to stimulation by cold. The presumption is allowable, 

 at least, that besides affording the animal protection from external heat 

 or cold, the nerve-end organs within these spots originate afferent 

 impulses which actuate a thermotactic center, some knot of neurones 

 which by its connection with the other centers of the brain controls all 



