22 THE FUNCTIONS OF ANIMALS. 



which animals are able to utilise. Thus most plants derive 

 the carbon they require from the carbonic acid gas of the 

 air, while only a few (green) animals have this power; 

 all the others depend for their carbon supplies on the 

 sugar, starch, and fat already made by other animals, or by 

 plants. As regards nitrogen, most plants take this from 

 nitrates and the like, absorbed along with water by the 

 roots; whereas animals obtain their nitrogenous supplies 

 from the complex proteids formed within other organisms. 

 Most plants, therefore, feed at a lower chemical level than 

 do animals, and it is characteristic of them that, in the 

 reduction of carbonic acid, and in the manufacture of 

 starch and proteids, the kinetic energy of sunlight is trans- 

 formed by the living matter into the potential chemical 

 energy of complex food stuffs. Animals, on the other 

 hand, get their food ready made ; they take the pounds 

 which plants have, as it were, accumulated in pence, and 

 they spend them. For it is characteristic of animals that 

 they convert the potential chemical energy of food stuffs 

 into the kinetic energy of locomotion and other activities. 

 In short, the great distinction — an average one at best — is 

 that most animals are more active than most plants. The 

 time-honoured "distinctions between plants and animals" 

 may be condensed in the opposite table. 



Chief functions of the animal body. — We have seen that 

 there are two master activities in animals, those of muscular 

 and of nervous structures, and that the other vital processes, 

 always excepting growth and reproduction, are subservient 

 to these. Let us now consider the various functions, as 

 they occur in some higher organism, such as man, reserving 

 comparative treatment for a subsequent chapter. 



Nervous activities. — Life has been described as consisting 

 of action and reaction between the organism and its en- 

 vironment, and it is evident that an animal must in some 

 way become aware of surrounding influences. In a higher 

 animal there are always parts which are specially excitable. 

 These are the sensory end-organs : the retina of the eye for 

 light, certain parts of the ear for sound, papilla? on the tongue 

 for taste, part of the lining of the nasal chamber for smell, 

 tactile corpuscles of the skin for pressure and temperature. 



All these end-organs are associated with nerves which 



