152 THE FROG chap. 



temperature of the body more than very shghtly above 

 that of the surrounding medium. In warm-blooded animals, 

 such as ourselves, the temperature 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. 1 1 ), may be looked upon as an ex- 

 cessive process of waste unaccompanied by repair. Owing 

 to tlie action of certain microscopic plants known as 

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

 jM'oteids 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. 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. 



I'K/VCTICAL UIKIiCTIONS. 



The Organs of Eespiration and of .Voice.— I'i" out a ftog in 



the usual way (pp. 31 and 32), lemove the heart, and make out the 

 precise relations of the lungs, first distending them with air through the 

 glottis. The specimen already used for the dissection of the vascular 

 system or alimentaiy can.al will serve the purpose. Harden thoroughly 

 in spirit and note ( l'"ig. 44) the laiyngo-traclii-al chninbcr, \\ hich communi- 

 cates with the pharyn.K through the glottis on the one hand, and on the 

 other with both lungs. Observe also the posterior horns of the hyoid 

 which embrace the glottis, and then separate them from the lar)'ngo- 



