62 DEVELOPMENT AND HEREDITY. 



diate causes of the motions, changes of shape, and 

 functions of animals, we find in like manner that it 

 is the nervous system which is implicated. If we 

 examine all the changes that take place in an animal, 

 it appears that before most of them occur, there is 

 a peculiar activity of the nervous system. What- 

 ever may be the primary source of the forces causing 

 the change, we know from the science of physiology 

 that every display of physiological forces, the per- 

 formance of every function, is subject to the control 

 of the nervous system. The swallowing of food, the 

 secretion of the juices which digest it, its propulsion 

 through the alimentary canal, the absorption of nutri- 

 ment through the walls of stomach and intestine, 

 the distribution of the nutriment throughout the 

 body, respiration and circulation, glandular secretion, 

 assimilation in the various parts of the body, repro- 

 duction, locomotion, muscular action, both voluntary 

 and involuntary, — all of these are caused by the 

 activity of the nervous system. A nervous disturb- 

 ance which is at first purely mental may check or 

 alter the whole nutrition and growth of a body, — as 

 when a person loses appetite and grows thin in con- 

 sequence of a severe mental shock or disappoint- 

 ment. While appetite or hunger is a sensation 

 localised in the stomach, in reality it is caused by 

 the excessive consumption of the nutri-ment in the 



