4:70 HANDBOOK OF PHYSIOLOGY. 



these routes become, to use a metaphor, well worn : there is, as it were, 

 a beaten track along which the nerve-force travels with much greater 

 ease than formerly: so much so that a slight stimulus, such as the pres- 

 sure of the foot on the ground, is sufficient to start and keep going in- 

 definitely the complex reflex actions of walking during entire mental 

 abstraction, or even during sleep. In such acts as reading, writing, and 

 the like, it would appear as if the will set the necessary reflex machinery 

 going, and that the reflex actions go on uninterruptedly until again in- 

 terfered with by the will. 



Without this capacity possessed by the nervous system of " organiz- 

 ing conscious actions into more or less unconscious ones/' education or 

 training would be impossible. A most important part of the process by 

 which these acquired reflex actions come to be performed automatically 

 consists in what is termed association. If two acts be at first performed 

 voluntarily in succession, and this succession is often repeated, the per- 

 formance of the first is at once followed mechanically by the second. 

 Instances of this " force of habit " must be within the daily experience 

 of every one. 



Of course it is only such actions as have become entirely reflex that 

 can be performed during complete unconsciousness, as in sleep. Cases 

 of somnambulism are of course familiar to every one, and authentic in- 

 stances are on record of persons writing, and even playing the piano dur- 

 ing sleep. 



4. Automatism. 



To nerve centres, it is said, belongs the property of originating 

 nerve-impulses, as well as of receiving them, and conducting and reflect- 

 ing them. 



The term automatism is employed to indicate the origination of ner- 

 vous impulses in nerve-centres, and their conduction therefrom, inde- 

 pendently of previous reception of a stimulus from another part. It is 

 impossible, in the present state of our knowledge, to say definitely what 

 actions in the body are really in this sense automatic. An example of 

 automatic nerve-action has been already referred to, i. e. , that of the 

 respiratory centre, but the apparently best examples of automatism are 

 found, however, in the case of the cerebrum, which will be presently 

 considered. 



5 and 6. Augmentation and Inhibition. 



Nerve-cells not only receive and reflect nerve impulses, and also in 

 some cases even originate such impulses, but they are also capable of in- 

 creasing the impulse, and the result is what is called augmentation; and 

 when a nerve-centre is in action, its action is also capable of being in- 



