ANIMAL HEAT. 339 



point below the origin of this nerve, all the respiratory 

 muscles may be paralyzed; the diaphragm is thus left to 

 work alone, and, in a case of necessity, it is, of itself, capable 

 of continuing respiration. 



II. ANIMAL HEAT. 



Our study of the phenomena of the lungs, the respiration 

 of the tissues, and the temperature of the blood, will enable 

 us to examine rapidly the question of animal heat, a question 

 the fundamental data of which we are already familiar with, 

 and which requires, for its completion, only a few special 

 details. 



It has long been known that the temperature of the supe- 

 rior animals is, up to a certain point, independent of the sur- 

 rounding or ambient temperature : these animals are said to 

 have a constant temperature ; the mammalia and birds be- 

 long to this class. In the other classes of the animal kingdom 

 the temperature of the body depends more or less on the 

 variations in the external temperature ; and the animals be- 

 longing to these are said to have a variable temperature. 

 The former have also, less happily, been called warm-blooded 

 animals, and the latter cold-blooded animals. 1 



1 ** Between the physiological properties of the muscles and 

 nerves of the warm-blooded and the cold-blooded animals differences 

 exist, which may, perhaps, be owing to the modifying influence of 

 the surrounding or ambient medium. Thus the muscles and nerves 

 of a torpid dormouse, or those of a rabbit under certain circum- 

 stances (subjected to gradually increasing cold) which make it 

 resemble a cold-blooded animal, are found exactly similar to those 

 of a frog or a tortoise during the winter. When animals are in a 

 state of torpor, the nervous excitation spreads slowly, and the con- 

 traction of the muscles lasts after the excitation of the nerve has 

 ceased, while in those which are not benumbed the contraction of 

 the muscles takes place rapidly at the moment of the excitation, 

 and ceases with it. The special modification produced by cold in 

 the muscles and nerves of animals may, however, be followed by 

 other results: In the warm-blooded animals the nerves and muse les 

 belonging to the system of the great sympathetic nerve, are found 

 endowed with the same properties as the muscles and nerves of the 

 cercbro-spinal system when benumbed. . . . This normal, or phy- 

 siological, torpor of the muscles and nerves is probably due to a 

 less perfect histological organization, accompanied by a lower de- 

 gree of excitability or irritability of the organized matter. ' ' ((Jl. 



