THERMOTAXIS 673 



sufficient stimulus; and it is a tempting hypothesis 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. 



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

 lation can be attributed to such stimuli. For it lias 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-production, 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. Injury to various portions of the cortex cerebri in 

 the dog and other animals, and lesions of the pons, medulla oblongata 

 and cord in man, may be followed by increase of temperature. 

 When the spinal cord is cut below the level of the vaso-motor centre, 

 the increased loss of heat from the skin due to dilatation of the 

 cutaneous vessels masks any increase of the heat-production which 

 may possibly have taken place, and the internal temperature falls ; 

 but if the loss of heat is diminished by wrapping the animal in 

 cotton-wool the temperature may rise. From such phenomena it 

 has been surmised that certain ' centres ' in the brain have to do 

 with the regulation of temperature by controlling the metabolism 

 of the tissues; that they cause increased metabolism when the 

 internal temperature threatens to sink, diminished metabolism 

 when it tends to rise. The cutting off, it is said, of the influence 

 of the ' heat centres ' by section of the paths leading from them 

 allows the metabolism of the tissues to run riot, and the temperature 

 to increase. But diversity of opinion still reigns as to the existence 

 or, if they exist, as to the precise location of the nervous centres 

 which preside over this function. It is stated by some workers 

 that animals deprived of the "cerebral hemispheres, the corpora 

 striata and one of the optic thalami, still ni;int,iin to a ivm .irk.tbK- 

 degree a body temperature independent of their environment, but 



