44 THE GRAMMAR OF SCIENCE 



of stored sense-impressions, no chain of what we term 

 thoughts intervening between the immediate sense- 

 impression and the message to the motor nerve. Thus 

 what we term consciousness is largely, if not wholly, due 

 to the stock of stored impresses, and to the manner in 

 which these condition the messages given to the motor 

 nerves when a sensory nerve has conveyed a message to 

 the brain. The measure of consciousness will thus largely 

 depend on (i) the extent and variety of past sense-impres- 

 sions, and (2) the degree to which the brain can perma- 

 nently preserve the impress of these sense-impressions, or 

 what might be termed the complexity and plasticity of 

 the brain. 



^ 3. — The Brain as a Central Telephone Exchange 



The view of brain activity here discussed may perhaps 

 be elucidated by comparing the brain to the central office 

 of a telephone exchange, from which wires radiate to the 

 subscribers A, B, C, D, E, F, etc., who are senders, and to 

 W, X, Y, Z, etc., who are receivers of messages. A, 

 having notified to the company that he never intends to 

 correspond with anybody but W, his wire is joined to W's, 

 and the clerk remains unconscious of the arrival of the 

 message from A and its despatch to W, although it passes 

 through his office.^ There is indeed no call-bell. This 

 corresponds to an instinctive exertion following uncon- 

 sciously on a sense-impression. Next the clerk finds by 

 experience that B invariably desires to correspond with 

 X, and consequently whenever he hears B's call-bell he 

 links him mechanically to X, without stopping for a 

 moment his perusal of Tit-Bits. This corresponds to a 

 habitual exertion following unconsciously on a sense- 

 impression. Lastly, C, D, E, and F may set their bells 

 ringing for a variety of purposes ; the clerk has in each 



1 If these wires were connected outside the office, we should have an 

 analogy to certain possibilities of reflex action, which arise from sensory and 

 motor nerves being linked before reaching the brain — e.g. a frog's leg will 

 be moved so as to rub an irritated point on its back even after the removal 

 of the brain. 



