380 THE NERVOUS SYSTEM. 



organs, or vice versa, three successive sets of nerve fibres take part in 

 its transmission. 



In each separate region of the cerebro-spinal system, furthermore r 

 the gray substance may act as a nervous centre, to transform sensitive 

 impressions into a motor impulse, and thus give rise to reflex action. 

 Such a reflex action may take place in the nervous centre nearest the 

 periphery, without calling into operation any other than its own special 

 endowments, and without presenting any character of consciousness 

 or volition. But the sensitive impression may also be transmitted 

 through the whole series of longitudinal connections to the cortex 

 of the brain ; and the motor impulse thus excited may pass in the 

 reverse direction through its entire route to the muscles of the limbs. 

 The convoluted layer of gray substance in the cerebral hemispheres 

 serves therefore as a great concave mirror, by which impressions 

 coming from without are received as conscious sensations and ideas, 

 and reflected in the form of intelligent, voluntary acts. To produce 

 this result, the whole mechanism of the cerebro-spinal system is called 

 into operation, each part acting in succession, to modify or transmit the 

 nervous impulse. 



