Chap. VII.] THE NERVOUS SYSTEM. 75 



nerve-cell, as being made up of a cell-body and its processes. 

 Inasmuch as the processes arise from the cell-body, the latter 

 is often spoken of as the " origin " of the fibres. By the origin 

 of a fibre, then, we mean the cell-body from which that fibre 

 springs. 



Like the muscle-cell the neurone is irritable and responds to 

 stimuli, but its mode of response is quite different from that of 

 the muscle. If a muscle be stimulated, changes occur in its sub- 

 stance, which changes result in the contraction of the muscle. 

 If, however, we stimulate a neurone, we find that although there 

 is no visible alteration in the part stimulated, yet a change in 

 the substance of the neurone takes place which passes along 

 throughout the entire neurone, and even to adjacent neurones, 

 and so on from neurone to neurone often for a great distance. 

 This invisible change which sweeps like a wave along the 

 neurone is called the " nerve-impulse " ; and the fundameyital 

 property of the neurone is to conduct nerve-impulses. 



We may roughly compare the passage of a nerve-impulse along 

 a neurone with the passage of the electrical current along a wire. 



The result of the stimulus depends not upon any peculiarity 

 of the neurone itself, but upon its anatomical relations to other 

 neurones, and to other tissues of the body. Thus, if impulses 

 be conducted to a muscle-fibre, the muscle contracts ; but if 

 they be conducted to the brain, we have as the result a con- 

 scious sensation. 



Under normal conditions an impulse always passes along a 

 neurone in the same direction, travelling towards the cell-body 

 by the dendrones, and away from it by the axones. Hence the 

 dendrones may be regarded as receiving processes ; the axones 

 as transmitting processes. 



The nervous system of man and of the higher animals has 

 been divided into the following parts : — 



1 



Nerves 



Tap a J ^P^°^^ \ Peripheral Nervous System. 



1 Sympathetic J 



Spinal Cord i 



C Medulla oblongata I Cerebro- spinal Axis, 



■o I Pons Varolii v or 



] Cerebellum I Central Nervous System. 



I Cerebrum j 



