152 THE FROG CHAP. 



Evolution of Heat. The oxidation of the tissues, like 

 that of coal or wood in a fire, is accompanied by a rise 

 in temperature. But in the frog, as in other cold- 

 blooded animals, the evolution of heat is never sufficient 

 to raise the temperature of the body more than very 

 slightly above that of the surrounding medium. In 

 warm-blooded animals, such as ourselves, the tempera- 

 ture is regulated, according to the season, by a greater 

 or less evaporation of water from the surface of the 

 body. In the frog this is not the case : the temperature 

 of the animal is always nearly the same as that of the 

 air or water in which it lives, and hence the frosts 

 of winter would be fatal to it, but for the habit of 

 hibernation (p. 8). 



Death and Decomposition. The decomposition under- 

 gone by a dead frog (p. u) may be looked upon as an 

 excessive process of waste unaccompanied by repair. 

 Owing to the action of certain microscopic plants known 

 as Bacteria, which will be referred to again in Part II. 

 (p. 257), the proteids undergo oxidation, amongst the 

 principal products of which are water, carbon dioxide, 

 ammonia, and certain gases of evil odour, such as 

 sulphuretted hydrogen and ammonium sulphide. Most 

 of the gases escape into the air, while the ammonia is 

 finally converted into nitrous and nitric acids. These, 

 combining with certain substances in the soil, give rise 

 to salts called nitrates and nitrites, which furnish one 

 of the chief sources of the food of plants. 



PRACTICAL DIRECTIONS 



The Organs of Respiration and of Voice. Pin down a 

 frog in the usual way (pp. 31 and 32), remove the heart, and 

 make out the precise relation of the lungs, first distending 

 them with air through the glottis. The specimen already 

 used for the dissection of the vascular system or alimentary 

 canal will serve the purpose. Harden thoroughly in for- 

 maline or spirit and note (Fig. 44) the laryngo-tracheal 



