64 PHYSIOLOGY CHAP. 



latter, represented by reptiles, amphibia, and fish, he called hetero- 

 thermic or poikilothermic animals. 



The former class maintain their average temperature within 

 certain narrow limits of variation, even when exposed to great 

 changes in the temperature of their environment. In order 

 to account for this relatively uniform temperature, we must 

 remember that when these animals are kept for a long time in 

 cold surroundings their production of heat is increased in order 

 to compensate for the loss of heat ; on the other hand, their produc- 

 tion of heat diminishes when they are kept in warm surroundings. 

 Their life is endangered when, owing to exposure to extreme 

 cooling or heating, a big fall or rise is produced in their 

 temperature. 



Poikilothermic animals, on the contrary, can endure without 

 any danger to life great changes in their temperature, produced by 

 variations in the temperature of the environment in which they live 

 or in which they are placed for the purpose of experiment. They 

 too, however, have a temperature of their own somewhat higher than 

 that of the environment ;" they too, like homoiothermic animals, 

 are capable in some small measure of reacting to the external 

 temperature by increasing or decreasing the processes of oxidation, 

 and hence the production of heat. 



There is a class of animals occupying a position midway 

 between the homoiothermic and the poikilothermic animals. The 

 so-called hibernating animals and new-born animals in general 

 form an exception in the group of mammals and birds. These 

 animals cannot maintain their temperature within normal limits 

 when that of the environment is considerably lowered. When 

 their temperature falls below 20 they fall into a state of lethargy 

 (Ch. Eichet). 



Variations in temperature are usually measured with thermometers. 

 There are various kinds of thermometers, but they are all based on the same 

 principle, i.e. that a determined quantity of the thermometric substance 

 varies in volume according to the temperature. Every increase in tempera- 

 ture causes an expansion or increase in volume. 



The only condition which is essential if a substance is to be regarded as 

 thermometric is that its volume shall not be the same in two different 

 states of heat. This is why water cannot be used. It therefore follows that 

 the greater the volumetric variations produced in the thermometer by heat, 

 the better will it answer its purpose. In other words, gases would be the 

 best substances for thermometry. Their use, however, has many practical 

 drawbacks, and for this reason the hydrogen thermometer is only used in 

 physics as a standard instrument. 



The thermometer most commonly used is one in which mercury serves as 

 the thermometric substance. For the measurement of very slight variations 

 of temperature there are thermometers (Baudin) which allow direct reading 

 to -fa of a degree and estimation to T ^. The attainment of such perfection 

 of course necessitates the use of a very large quantity of mercury the bulb 

 is therefore large and of a very slender tube in which the column of mercury 

 rises and falls. 



