20 THE FUNCTIONS OF ANIMALS. 



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 functions, always excepting 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 environment, and it 

 is evident that an animal must in some way feel, or become 

 aware of surrounding influences. In a higher animal we 

 find 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, papillae 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 are 

 stimulated by the excitation of the end-organ, and conduct 

 the stimulus inwards to what are called centres or ganglia. 



In Vertebrate animals the brain and spinal cord contain a 

 series of such centres, some of which serve for the per- 

 ception of the changes produced in the end-organs by the 

 stimulus, while others preside over the activities of the 

 muscles. I As we ascend in the scale we find that in addition 

 the brain possesses, to an increasing extent, the power of , 

 correlating present and past experiences, and originating or 

 inhibiting action in accordance with the judgment formed. J 



Thus, nervous activities involve (a) end-organs or sense 

 organs ; () centres or ganglia ; and (c) the conducting nerves, 

 some of which are afferent (or sensory) passing from end- 

 organs to ganglia, while others are efferent (or motor) 

 passing from centres to muscles. And in whatever part 

 there is activity there is necessarily waste of complex sub- 

 stances and some degree of exhaustion. 



/ It is interesting to notice, as a triumph of histological technique, 

 that Hodge, Gusfav Mann, and others have succeeded in demonstrat- 



