124 PSYCHOBIOLOGY 



remaining system, and still function, the function would possibly be ac- 

 companied by consciousness; two streams of consciousness might (by this 

 hypothesis) go in connection with the same individual. Such a condition 

 seems to be found in certain cases of hysteria. This is, however, a matter 

 which is open to different interpretations, and is not within the range of 

 the present discussion. 



SERIAL HABITS. 



The most striking characteristic of the nervous system as a whole, is 

 its capacity to form and retain habits. Using the term " habit " in the 

 usual sense, the nervous system is the only part of a complex animal 

 organism which does form habits. Assuming the possibility of forming 

 habits, (which is an empirical datum), the manner in which they are 

 formed and improved is easily understood on the reflex-arc hypothesis. 



Every reflex, which terminates in activity of striped muscle, tends to 

 initiate a new reflex, since the muscular activity, which is the termination 

 of the one reflex, tends to irritate receptors in the muscle itself. Reflex 

 activity once set up tends to continue until it is drafted off to effectors from 

 which no immediate afferent return is made, i. e., glands and possibly 

 also smooth muscles. 



The efferent direction of the discharge from muscular receptors, what- 

 ever its natural (i. e., instinctive), tendency, is modified by the general 

 perceptual reflexes simultaneously present, in accordance with the general 

 principles of drainage. Given a succession of two perceptual reflexes, 

 not previously connected with each other in an especial way, the emyotic 

 current resulting from the first will be drained into the admyotic current 

 of the second, thus setting up an actual arc between the two muscular 

 activities. If this succession is repeated a number of times (or even if, 

 under special conditions, it occur but once), a habit is formed and the 

 subsequent occurrence of the first reaction will tend to cause reflexly the 

 second reaction without the intervention of the second specific stimulus. 

 In this way, a long series of reactions, each of which originally depended 

 on a separate stimulation, may become serially connected and follow ac- 

 curately from the stimulus of the first one. If each link in the chain is 

 " conscious ", i. e., if it integrates the total nervous system to a sufficient 

 extent to serve as the physiological concomitant of an idea, the repetition 

 of this series is associative thought; and its formation is the association 

 of ideas. 



In accordance with the above scheme, all thought would seem to depend 

 on muscular activity sufficient in its degree to irritate muscular receptors. 

 More or less muscular activity demonstrably does accompany all thought 



