THE CEREBRO-SPINAL AXIS 235 



strength, time and duration of application, etc., can be accurately 

 governed. 



It has been noticed that the uninterrupted flow of an electric 

 current through a nerve is unattended by muscular contraction ; 

 it has likewise been seen that very slow changes in the strength 

 of the current are similarly unaccompanied by the manifestations 

 of ordinary stimulation; but sudden changes in the strength, 

 whether in the direction of increase or decrease, act as stimuli. 

 However, while the passage of a constant current through a nerve 

 does not manifest itself by contractions except at making and 

 breaking, such a passage brings about a change in the tissue of the 

 nerve known as electrotonus. It may be considered a state of 

 electric tension. In the anodic area the excitability is diminished 

 (anelectrotonus) ; in the kathodic area it is increased (katelectro- 

 tonus). Nor is the electrotonic condition restricted to that por- 

 tion of the nerve between the poles. Between the poles there is a 

 point where the two influences anelectrotonus and katelectro- 

 tonus meet and there is neither increased nor decreased excit- 

 ability. With weak currents this point is nearer the anode; 

 with strong ones nearer the kathode. A descending current 

 diminishes the excitability of a nerve; an ascending increases it. 

 Prolonged application of electric stimuli will exhaust nervous 

 excitability, but it may be restored by rest, or more quickly by an 

 opposite current. 



THE CEREBRO-SPINAL AXIS. 



The cerebro-spinal axis embraces the nervous matter in the 

 cranial cavity and in the spinal canal, excepting the roots of the 

 cranial and spinal nerves. This axis consists of both white and 

 gray matter. The white matter is made up of conducting ele- 

 ments; the gray matter consists of a number of connected ganglia. 

 In the cord the white matter is situated externally; in the brain 

 the gray. The encephalon is situated in the cranial cavity 

 and consists of the cerebrum, the cerebellum, the pons Varolii, 



