

THE CENTRAL NERVOUS SYSTEM. 43 



between the spinal cord and the ganglion, stimulation of the peripheral 

 portion will have no result, either as regards sensation or muscular or 

 glandular activity. Stimulation of the central portion will, however, 

 be followed by sensation, and may result in reflex muscular movements. 

 If the anterior root be divided, stimulation of the peripheral portion 

 will be followed by muscular movements, while stimulation of the 

 central end will give rise neither to sensation nor reflexes. In the 

 production of a simple reflex by stimulation of the central end of the 

 divided posterior root, the impulse passes from the terminal arborisa- 

 tion of the fibre or of its collaterals in the grey matter of the spinal 

 cord across a synapse to the dendrons of a second neuron, the axon of 

 that neuron passes it on to the dendrons of one or more of the multipolar 

 cells the axons of which constitute the anterior nerve roots, and the 

 impulse, altered in character, is thus transmitted to the responding 

 muscles. There is thus a law of conduction, called the " law of forward 

 direction," according to which an impulse will pass across a synapse 

 from the axon of one neuron to the dendrons of another, but not in 

 the reverse direction. 



The Function of the Cell. In a reflex action the afferent impulse 

 is usually greatly modified in its passage through the central 

 nervous system. For example, the comparatively slight stimulus of 

 a crumb in the larynx may be followed by violent coughing, accom- 

 panied by contraction of other muscles besides those concerned with 

 expiration. Again, when a reflex movement is excited by stimulation 

 of an afferent nerve, the impulses travelling along the efferent nerve 

 have a rhythm which is independent of that of the exciting stimulus. 

 It was formerly thought that these and other modifications of the 

 impulses in the nervous system were brought about by the nerve cell. 

 It has been shown,, however, in certain invertebrates that reflex action 

 can still take place for a short time when the cells associated with the 

 fibres forming the reflex arc have been destroyed. This and other 

 similar observations indicate that the characteristic features of reflex 

 action must be attributed not to the nerve cell but to the synapse. 

 The^ f unctionof the_jierve-cell is purely nutritive. 



The Function of the Axon. The function of the axon is most 

 conveniently studied in the spinal nerve trunks. These contain true 

 axons, which arise from the multipolar cells in the grey matter of 

 the spinal cord, as well as afferent fibres in connection with the cells of 

 the ganglia of the posterior roots of the spinal nerves, the latter not 

 being axons in the restricted sense of the term, but showing no 

 difference in function from the axons proper except as regards the 

 direction in which they normally conduct impulses. 



