ANIMAL HEAT 595 



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 tempera- 

 ture (Chap. XIII.). A change of temperature is their ' ade- 

 quate ' and sufficient stimulus ; and it is a tempting hypothesis 

 that these are the afferent nerves concerned in the reflex regula- 

 tion 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. 



It is more than doubtful, however, whether the whole chemical 

 regulation can be attributed to such stimuli. For it has been 

 found that the relation between heat -production and extent of 

 surface in animals (guinea-pigs) of different size is unaltered when 

 the air temperature is made so nearly the same as that of the skin 

 that the temperature nerves can hardly be supposed to be excited. 



There is some evidence that the bioplasm the living substance of 

 different animals, even when the external conditions are the same, 

 may differ specifically in the average intensity of metabolism to 

 which it is pitched. When exposed to a temperature about equal 

 to that of warm-blooded animals, the green lizard (Lacerta viridis] 

 and the bull-frog, which live in the temperate zone, and for which 

 a temperature of 37 C. is highly abnormal, double their heat -pro- 

 duction, and soon die. Tropical poikilothermal animals, such as the 

 alligator, also double their heat-production, but the highest values 

 reached are only one-half that of the lizard at 25 C. Apparently 

 the bioplasm of the tropical animals has adapted itself to a high 

 external temperature, and works very economically even at the 

 highest temperatures (Krehl). 



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 in a few hours by a rise of temperature in 

 the rectum (i to 2), and still more in the duodenum, which is nor- 

 mally the hottest part of the body in this animal. The heat-pro- 

 duction, the respiratory exchange, and the nitrogen excretion are 

 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 

 temperature has returned to normal, a fresh puncture may again 

 cause a rise. 



Some observers hold that the chief seat of the increased metabolism 

 is the skeletal muscles, others the liver. The question turns 

 largely upon the success of the puncture experiment after the 

 previous administration of curara on the one hand, and of strychnine 

 on the other. For curara cuts out the motor innervation of 

 the skeletal muscles, and strychnine convulsions exhaust the 



38-2 



