(11. VlirJ THE XERVOrS SYSTEM 95 



The fiuictiou of an axon is to convey a nerve impulse. The 

 j)recise nature of such an impulse is still ven' obscure, but it 

 appears to be a reversible phjsico-cheniical jDrocess unassociated, 

 so far as is known, with metabolic change. An impulse maj' be 

 started artificially (as b}' stimulating Mith an electric current) 

 at any part of a neuron, and if this is done it travels to every 

 other part of the neuron. In normal life the impulse is started 

 at one end of the neuron (being transmitted from the adjoining 

 neuron), and travels to the other end, but in the case of accidents 

 it ma}' start at any j^oint. Some slight expenditure of energy 

 is required to set it up at the stimulated spot, but when once 

 this is done, if the stimulus is sufficiently strong, it causes a 

 propagated disturbance without being attended by any con- 

 sumption or evolution of energy. 



It is knoA^Ti that a nerve impulse is accompanied by an 

 electrical change in the neuron concerned, and by taking advantage 

 of this fact, the rate of transmission of an impulse can be measured. 

 A stimulus is applied at one end of a long nerve, and the electrical 

 condition of the nerve is recorded by a galvanometer at the other 

 end. In this way it has been found that an impulse travels at 

 the rate of twenty feet per second. 



When an axon is cut the part still connected with the cell 

 survives, the severed portion alone undergoing degenerative 

 changes, the subsequent observation of which may serve as a 

 guide to the destination of particular fibres. 



The nervous system is composed of (1) the brain and spinal 

 cord (together constituting the central nervous system), and 

 (2) all the other nerves of the body, these forming collectively 

 the peripheral nervous system. Impulses are of two principal 

 kinds, those passing into the central nervous system (or afferent 

 impulses) and those passing out (or efferent impulses). These 

 two chief kinds of impulses are never transmitted along the same 

 neurons. 



An afferent or sensory impulse starts at some special sense 

 organ and is conveyed along a neuron, whose nerve cell is small 

 and placed in a ganglion (a collection of nerve cells) outside of but 

 not far from the brain or spinal cord. The particular impulses 

 differ from one another in that each starts from a different 

 position and is carried to its own special destination in the 

 central nervous system. The skin is full of such end organs, and 



