412 THE NERVOUS SYSTEM 



delicacy of the nervous structiire and the difficulties m 

 the way Of experiment our physiological knowledge of 

 the nervous system is extremely limited, and, in fact, 

 much has still to be learned of the physiology of nerves 

 and nerve cells. In the following pages we shall simply 

 trj^ to indicate a few of the facts which have been deter- 

 mined up to the present time, the manner in which such 

 knowledge has been acquired, and its bearing upon our 

 common activities. 



Action, voluntary and reflex. — The action of every 

 organ of our body is dependent upon nerve control. The 

 simplest division that can be made of the actions of 

 the body separates them into two classes. Those which 

 require the exercise of the will and are associated with 

 consciousness are called voluntary actions. Those which 

 take place without any effort of will and may or may not 

 be accompanied by consciousness we call reflex. All such 

 actions as the picking up of objects Mith the fingers and 

 the avoiding of obstacles in walking, in short, all actions 

 which involve a conscious choice on our part belong to 

 the voluntary type. All such actions as the regulation 

 of the beating of the heart, the withdrawal of the hand 

 from a hot object, etc., in which the action takes place 

 without any consciousness of choice on our part, belong 

 to the class of reflexes. 



The relation which consciousness may bear to such 

 actions is brought out in the two examples of reflex 

 action cited. In the beating of the heart consciousness 

 is entirely lacking; while in the withdrawal of the burned 

 hand we may be conscious of the pain and the move- 

 ment, but ordinarily the movement has occurred before 

 the pain is felt, and hence, in this case, while conscious- 



