The Nervous System. 273 



the assistance of the brain ; and it preserves by its auto- 

 matic action both muscular and arterial tone. In speaking 

 of the automatic action of the spinal cord, it is necessary 

 to remember that it can originate no impulses in the 

 same way that the brain does, and that automatic action in 

 the cord is due to afferent impulses. 



The cord is not one large centre, but a series of centres 

 lying end to end, each capable to a greater or less extent of 

 acting independently of its neighbour, and each centre pos- 

 sessing its afferent and efferent roots. The white substance 

 may be regarded as the communicating paths between the 

 cord and the brain ; the grey matter as essentially the seat 

 of reflex actions. Experiment has shown that both the grey 

 and white matter is devoid of sensibility. The ganglion 

 on the superior root of the spinal nerve is not a centre for 

 automatic or reflex action, and the same may be said of all 

 other ganglia, cerebro-spinal or sympathetic. 



So long as the pathways to the brain are open to sensory 

 and motor impression, so long is the animal capable of per- 

 forming movements and recognising sensation ; but if by 

 operation or accident a part or the whole of the cord be 

 severed, certain symptoms of what we speak of as paralysis 

 appear ; whether this occurs to the right or left of the body 

 depends upon the part injured. If the injury be inflicted 

 above the decussation, at the extreme superior part of the 

 cord, a right cord lesion leads to paralysis on the left side, 

 both sensor}^ and motor ; if below the decussation, the motor 

 paralysis corresponds to that side of the cord injured, and 

 loss of sensation occurs on the opposite side (see Fig. 26;. 

 Loss of motion may not occur immediately after an accident 

 or injury to the spinal cord ; but this is accounted for by the 

 fact that the movements observed are not those performed 

 through any intelligent action on the part of the animal, but 

 by reflex action through the spinal centres below the injury. 

 These reflex actions are seen very perfectly in the frog, 

 where after destruction of the brain, some of the actions 

 appear to be produced as if by reason or consciousness. 

 The most remarkable of these experiments is that per- 

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