328 THE BODY AT WORK 



the pupil, constricts the bloodvessels of the ear, erects the hairs of 

 the head, as if to the manner born. To take another example, 

 in a monkey the two nerves supplying respectively certain 

 flexor and certain extensor muscles of the forearm were cut, 

 and their ends crossed, so that flexor nerve-fibres grew down to 

 extensor muscles, and extensor fibres to flexor muscles. There 

 was no bungling of reflex actions or of voluntary actions when 

 the new roads were first used. The monkey did not jerk its 

 hand open when it tried to scratch or to grasp a nut. 



When experimental data first began to accumulate, physi- 

 ologists drew diagrams and made models of the nervous system 

 in which they represented it as composed of conducting arcs. 

 The arcs were superposed to indicate that they were of various 

 grades spinal for ordinary reflexes, bulbar for co-ordinated 

 actions, through the grey matter in the centre of the great 

 brain for " ideo-motor " actions, through the cortex of the great 

 brain for voluntary acts. They spoke of authority and re- 

 sponsibility, comparing the nervous system to an army or a club. 

 It is premature to attempt a theory of the nervous system 

 compatible with recent discoveries regarding its structure and 

 mode of working, but it is clear that the diagrams and 

 metaphors to which we have just referred were misleading. 

 In place of attempting to disarticulate the machine, we ought 

 to emphasize its structural unity. The results obtained by 

 uniting heterologous nerves cannot be explained by reference 

 to a model made of wires and pieces of cork. They do not fit 

 in with any organization of human units or with any postal 

 system or telephonic apparatus for transmitting news. Prob- 

 ably the lines of thought which will prove most fruitful are 

 somewhat as follows : (1) An efferent discharge occurs as the 

 result of the opening of a circuit from a muscle back to the 

 muscle. Afferent impulses call them sensory, on the under- 

 standing that this does not imply that they appear in con- 

 sciousness are ceaselessly flowing from receptors to effectors in 

 the muscle. A sensation in the case of skeletal muscles 

 usually a skin sensation reinforces them to discharging- 

 point. If the spinal cord has been severed from the brain, the 

 up-and-down flow does not reach beyond its grey matter. It is 

 short-circuited. If the brain is in normal connection with the 

 spinal cord, sensory impulses travel upwards to its cortex (with- 



