ORGANS OF THE SENSORI-MOTOR SYSTEM 109 



All neurones are polarized — that is, the nervous impulse that 

 traverses them goes in one direction only, as indicated by the 

 arrows in the figures. At 2, for instance, some physical agency 

 affects a receptor, or nerve-termination, or bunch of dendrites, 

 as shown by the short arrow. The physical disturbance in the 

 receptor then initiates another physical disturbance, or nervous 

 impulse, in the axon and this travels along the latter with a 

 velocity of about 40 to 100 metres per second until it reaches 

 the nerve-cell. Passing through the latter it breaks up in another 

 bunch of dendrites somewhere in the grey matter of the 

 ganglion, or part of the spinal cord. No observation suggests 

 that the direction of a nervous impulse in an axon is ever 

 reversed. 



Now, whatever they may be, all units in the nervous system, 

 peripheral or central, are made up of neurones and a neurone 

 is always a nerve-cell with its dendrites. One set of dendrites 

 is afferent, that is, impulses pass through them into the cell. 

 The other set of dendrites is efferent, that is, impulses pass into 

 them out of the cell. A nervous path in the nervous system 

 consists a ways of several or very many neurones placed end to 

 end. 



40^. The Nervous Impulse. We do not know what pre- 

 cisely is a nervous impulse. It is certainly a physical disturbance 

 established by the stimulation of a receptor organ and then 

 communicated to the materials of an axon. A nervous impulse 

 is accompanied by an electric disturbance, in this way, — let 



be a small part of an axon and let the arrows 



indicate that an impulse travels along the axon, or fibre, from 

 left to right. As the impulse passes each small segment, say 

 «', of the axon, that segment becomes electrically negative with 

 respect to the adjacent parts of the axon, w^hich are positive. 

 The impulse may be compared with the current passing along 

 an electric conductor, with the flash that passes along a train 

 of gunpowder which is fired, with the jolt which passes along a 

 train of wagons when the engine suddenly starts, with the 

 wave of vibration transmitted along a rope when one end of 

 the latter is twitched, etc., but it is none of these things. From 

 our present point of view a nervous impulse conveys a stimulus 

 that originates somewhere by a change in a receptor organ. 



