500 NUTRITION. 



vasomotor action which, by constricting the cutaneous vascular areas, or by 

 dilating the splanchnic vascular areas, causes a smaller flow through the skin,, 

 and a larger flow of blood through the abdominal viscera, will tend to heat 

 the body. In the second place, besides this, the special nerves of perspiration 

 will act directly as regulators of temperature, increasing the loss of heat 

 when they promote, and lessening the loss when they cease to promote, the 

 secretion of the skin. The working of this heat-regulating mechanism is 

 well seen in the case of exercise. Since every muscular contraction gives 

 rise to heat, exercise must increase for the time being the production of heat; 

 yet the bodily temperature rarely rises so much as a degree centigrade, if at 

 all. By exercise the respiration is quickened, and the loss of heat by the 

 lungs increased. The circulation of blood is also quickened, and the cuta- 

 neous vascular areas becoming dilated, a larger amount of blood passes 

 through the skin. Added to this, the skin perspires freely. Thus a large 

 amount of heat is lost to the body, sufficient to neutralize the addition 

 caused by the muscular contraction, the increase which the more rapid flow 

 of blood through the abdominal organs might tend to bring about being 

 more than sufficiently counteracted by their smaller supply for the time. 

 The sense of warmth which is felt during exercise in consequence of the 

 flushing of the skin, is, in itself, a token that a regulative cooling is being 

 carried on. In a similar way the application of external cold or heat defeats 

 its own ends, either partially or completely. Under the influence of external 

 cold, the cutaneous vessels are constricted, and the splanchnic vascular areas 

 dilated, so that the blood is withdrawn from the colder and cooler regions to 

 the hotter and heat-producing organs. This vascular change may be used 

 to explain the fact that stripping naked in a cold atmosphere often gives rise 

 to a distinct increase in the mean temperature of the blood, as indicated by 

 a thermometer placed in the mouth, though possibly the effect may be partly 

 due to an actual increase of the production of heat. Under the influence of 

 external warmth, on the other hand, the cutaneous vessels are dilated, a rapid 

 discharge of heat takes place ; and if the circumstances be such that the body 

 can perspire freely, and the perspiration be readily evaporated, the tempera- 

 ture of the body may remain very near to the normal, even in an excessively 

 hot atmosphere. Thus, more than a century ago, two observers were able 

 to remain with impunity in a chamber heated even to 127 C. (260 Fahr.), 

 and with ease in one so hot that it became painful for them to touch the 

 metal buttons of their clothing. It is unnecessary to give any more exam- 

 ples of this regulation of temperature by variations in the loss of heat ; they 

 all readily explain themselves. 



447. The production of heat, its variations and regulation. As we 

 have already said, the exact determination of the amount of heat produced 

 in the living body is attended with great difficulties ; still, certain conclu- 

 sions have been arrived at based partly on direct calorimetric observations, 

 the more recent ones with improved calorimeters being especially valuable, 

 and partly on what seem to be trustworthy deductions from observed chemi- 

 cal changes. 



The rate of production of heat in a living body is determined by a 

 variety of circumstances. In the first place, what may be called the general 

 rate of metabolism, and so of the production of heat, varies in different 

 kinds of animals. Of two animals of the same bulk and weight placed under 

 the same circumstances, one " living faster " than the other, metabolizes its 

 living substance more rapidly, and so produces heat more rapidly. Thus 

 direct calorimetric observations, as far as they at present go, show that a 

 man, on the average, produces more heat, per kilo, per hour, than does a 

 dog, and a dog more than a rabbit. Probably every species has what may/ 



