SECTION III. 

 THE CENTRAL NERVOUS SYSTEM. 



CHAPTER X. 

 R*EFLEX ACTION IN THE FROG. 



In a nerve fibre impulses are transmitted with equal facility in 

 either direction (Exp. 18, p. 45), but in a reflex arc they go in one 

 direction only. The nerve centre is responsible for this directive 

 influence, and the centre may be situated either peripherally, as in 

 ganglia, or centrally, as in the brain and spinal cord. The exact 

 significance of the centres in the brain and spinal cord varies con- 

 siderably in different animals according to their degree of develop- 

 ment. In a general way the centres in the spinal cord are primarily 

 the local centres for each segment of the body, but the reflex 

 activities of other segments may be caused to cooperate so as to 

 bring about complex movements. The centres in the brain, on the 

 other hand, are to be regarded as affording higher nervous path- 

 ways which the reflex impulse does not necessarily traverse, but 

 in which, when it does so, considerable modification may occur. 

 The impulse may, for example, become suppressed (inhibited) or 

 exaggerated (augmented) by its passage through nerve centres in 

 which memory impressions have been stored away. If these 

 memory impressions indicate that reflex movements would be 

 harmful to the animal then the movements may be inhibited: under 

 the converse conditions thay may be augmented. On account of 

 this dominating influence of the higher over the lower centres it is 

 clear that a precise analysis of the physiology of reflex action 

 demands a simplification of the experimental conditions, by iso- 

 lating the spinal cord from the brain. In the lower vertebrates 

 this can readily be done by actually removing or destroying the 

 latter. In the higher vertebrates, this operation is incompatible 

 with life, so that the brain is merely isolated from the spinal cord 

 by cutting the latter below the level of exit of the nerves to the 

 chief respiratory muscles (diaphragm). This so-called SPINAL 



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