THE NEKVOUS SYSTEM 329 



out, save in exceptional instances, arousing consciousness, or, 

 as we should prefer to express it in this connection, without 

 attracting attention) to a degree which varies with the several 

 classes of receptor and with the animal. A monkey reduced to 

 the condition of a " spinal animal " i.e., with its spinal cord 

 severed from its brain is less competent than a dog, and a man 

 is far less competent than a monkey. In other words, a man 

 habitually uses his brain more than does a monkey, and a 

 monkey more than a dog. The proportion which brain-weight 

 bears to body- weight roughly indicates the part the brain plays 

 in conducting the traffic of the body. (2) Communication within 

 the nervous system is almost unrestricted. If, before the 

 median nerve was divided at the wrist, receptor A usually 

 initiated a current which passed through the circuit to 

 effector X, and receptor B to effector Y, and if the new fibres 

 which grew downwards lost their way so that the one which 

 used to receive messages from A attached itself to B, and the 

 one which used to transmit commands to X attached itself to 

 Y, A is not thereby cut off from X, or B from Y. Such a 

 mechanical association is restricted to our diagrams. It does 

 not enter into Nature's plan. The spinal cord is not scored 

 with unchangeable paths. A messenger from A could always 

 reach either X or Y. It was not the path, but the struggle 

 with competing messengers, which directed him to X. 



When we endeavour to picture the mechanism of the nervous 

 system, we find ourselves faced by phenomena which appear 

 irreconcilable. One set of observations leads to the conception 

 of closed paths ; another set points to an open conductor. 

 The experimental crossing of nerves to which we have just 

 alluded shows that the nervous system is adaptable, to a degree 

 which seems extraordinary to anyone who attempts to compare 

 it with any of Man's devices for establishing communica- 

 tion. Paths appear to make themselves. On the other hand, 

 the more important, and therefore dominant, reflex actions, 

 such as swallowing, breathing, the maintenance of position, 

 are due to the union of receptors and effectors by lines which 

 are either reserved for their sole use, or, if shared by other 

 currents, it is on the understanding that they have a first 

 and altogether prepotent claim. No competing impulses can 

 divert them or block their way. All reflexes which in the 



