DEVELOPMENT OF HEAT REGULATION. 865 



as do normal pigeons. The rapid rise in temperature which occurs 

 when a hibernating marmot awakes, is not prevented by removal of the 

 cerebral hemispheres. 



An impartial examination of the above evidence leads to the verdict 

 that the existence of the so-called " heat centres " in the brain has not 

 been proved. In the first place, the results, even in the hands of the 

 same experimenter, are inconsistent ; some observers obtain exactly 

 opposite effects from apparently similar lesions. Further, in many 

 cases rabbits have been used for these experiments, and it is notorious 

 that their temperature is liable to considerable variations during 

 operative procedures. Even if the existence of these centres be 

 granted, even if it be allowed that after puncture there is an increase in 

 metabolism and in the production of heat, it by no means follows that 

 the centres are special centres for the regulation of temperature, and 

 give off " thermic nerves." 



It seems more probable that the mechanism of heat regulation 

 has the same cerebral representation as the voluntary muscles. In 

 the lower warm-blooded animals the representation of these in the 

 cerebral cortex is not well developed, and it has likewise been shown 

 that the removal of the cortex in them has little or no effect upon 

 the regulation of temperature. 



THE DEVELOPMENT OF THE POWER OF MAINTAINING A CONSTANT 



TEMPERATURE. 



In the cold-blooded animals there are traces of the power of maintaining 

 a constant temperature, as shown by the high temperature which a female 

 python is able to maintain for many weeks when she is incubating her eggs. 

 This instance is the more remarkable because during that time the python 

 takes no food or exercise. Further instances have already been mentioned in 

 the case of bees, and some species of fish. 



It is possible to trace in the warm-blooded animals the gradual develop- 

 ment of this power of regulation. Thus, during the development of a chick 

 there is first a stage in which the embryo responds to changes of temperature 

 in a similar manner to that of a cold-blooded animal ; then a stage of transition 

 in which there is a regulation for moderate changes of temperature ; and 

 finally, when a chick is hatched, the power of regulation resembles that of 

 a warm-blooded animal. 1 In 1824, Edwards 2 pointed out that young 

 mammals and birds may be divided into two classes, the warm-blooded 

 and the cold-blooded, according as they are, or are not, able to maintain their 

 temperature when removed from the warmth of the parents. The difference 

 lies in the relative development of the two classes active young animals 

 covered with fur or feathers, as in the case of the guinea-pig and chick, belong 

 to the former class ; while young animals born naked, blind, and helpless, 

 belong to the cold-blooded group. The inability to maintain a constant 

 temperature is due to diminished production of heat on exposure, and only 

 secondarily to excessive loss of heat. It has recently been shown that the 

 chick and guinea-pig can at birth regulate their production of heat, that young 

 cold-blooded mammals and birds are able to regulate only for moderate changes 

 of external temperature ; for, when exposed to cold, their temperature and 



1 Pembrey, Gordon, and Warren, Journ. PhysioL, Cambridge and London, 1894-95, 

 vol. xvii. p. 331. 



2 " De I'influence des agens physiques sur la vie," 1824. 



VOL. I. 55 



