450 BIOLOGY. 



state of tension, and hence keeping the processes of their 

 system in constant repair. 



2733. The rationale of this resides in the two nervous 

 substances having separated into ganglia and plexuses. 



2734. This also serves to explain another phenomenon, 

 viz. that the nerves indeed, but never the brain, attain to 

 perfect rest ; because the former is without ganglionic or 

 cortical substance, the latter is throughout surrounded, 

 and obviously interwoven, by it. 



2735. Therein lies the reason why the viscera do 

 not sleep. 



Sleep. 



2736. The condition of the nervous system, as hitherto 

 represented, is called that of being awake. It is the 

 interlude played by the nerves with the world, and with 

 the animal body. 



2737. When in a state of health the first interlude 

 ceases, then the other also is over or past. The world- 

 nerves, however, operate only upon the animal systems, 

 upon the senses and the motion of the muscles ; it could 

 therefore be these only wherein the nervous function, in 

 compliance with this cessation, is suppressed. Now, 

 muscular rest originates through suppression of the 

 tension between nerve and muscle. In sleep this rest 

 is also derived from the same means. 



2738. The tension between nerve and muscle can 

 only cease, if the tension also between the termination of 

 the brain and nerves be suppressed. We are now reduced 

 simply to the consideration of the nervous system, and 

 may, in treating of sleep, pay no attention to the muscular 

 system. 



2739. But whence comes the tension in the motor 

 nerves ? Obviously only from the cerebral tension. This 

 can originate only in two ways ; either through the 

 special organization of the brain, one or the other 

 substance being preponderant, or through the influence 

 of external stimuli. 



2740. The encephalic substance becomes more potent 

 than usual, if by rest the cortical substance becomes 



