5 i 4 A MANUAL OF PHYSIOLOGY 



mental results nor general reasoning have as yet shown 

 that in man, either in the tropics (Eykman) or in the north 

 temperate zone (Loewy), the chemical tone is diminished 

 by a rise of external temperature much above the mean of 

 an ordinary English summer, apart from the effect of the 

 muscular relaxation which heat induces. In a man, indeed, 

 at rest in a hot atmosphere, the production of carbon dioxide 

 and consumption of oxygen are, if anything, greater than 

 at the ordinary temperature. The regulation of tempera- 

 ture in an environment warmer than the normal seems, in 

 fact, to be brought about more by an increase in the loss 

 than a decrease in the production of heat. Evaporation 

 from the skin and lungs is an automatic check upon over- 

 heating as important as the involuntary increase of meta- 

 bolism upon excessive cooling. 



While it is known that the skeletal muscles, and perhaps 

 the glands and other tissues, are at one end of the reflex arc 

 by which the impulses pass that regulate the temperature 

 through the metabolism, we are as yet ignorant of the 

 precise paths by which the afferent impulses travel, of the 

 nerve-centres to which they go, and even of the end- organs 

 in which they arise. There are nerves in the skin which 

 minister to the sensation of temperature (Chap. XIII.). A 

 change of temperature is their * adequate ' and sufficient 

 stimulus ; and it is a tempting hypothesis, though nothing 

 more, that these are the afferent nerves concerned in the 

 reflex regulation of temperature that impulses carried up 

 by them to some centre or centres in the brain or cord are 

 reflected down the motor nerves to control the metabolism 

 of the skeletal muscles, and down the vaso-motor nerves to 

 control the loss of heat from the skin. 



Heat Centres. It is known that certain injuries of the central 

 nervous system are related to disturbance of the heat-regulating 

 mechanism. Puncture of the median portion of the corpus striatum 

 in the rabbit by a needle thrust through a trephine hole in the skull 

 is followed by a rise of temperature in the rectum (i to 2), and still 

 more in the duodenum, which is normally the hottest part of the body 

 in this animal. The heat-production and respiratory exchange are 

 also increased. These phenomena may last for several days (Ott, 

 Richet, Aronsohn and Sachs), and are due to stimulation of the 

 portions of the brain in the immediate neighbourhood of the injury. 

 Electrical stimulation of this region has a similar effect. When the 



