CAPACITIES OF NERVE-CELLS. 267 



ference in the behaviour of these organs from that of 

 ordinary nerve-fibres, and then regard all not appertain- 

 ing to the nerve-fibres as peculiar to the nerve-cells. 



We know that the nerve-cells are irritable, that 

 they transmit the excitement which arises in them, and 

 transfer it at the terminal organ. The excitement can 



O 



never occur of itself in a nerve-fibre, but it always re- 

 sults from an irritant acting externally, and can never 

 pass from one nerve-fibre to another, but always remains 

 isolated in the excited fibre. 



But where nerve-cells occur, the case is different. 

 As long as a nerve-fibre passes uninjured from the brain 

 and spinal-marrow, or from one of the accumulations of 

 nerve-cells situated in the periphery, to a muscle, ex- 

 citement arises without externally visible cause, and 

 this acts through the nerve on the muscle, sometimes at 

 regular intervals independently of the will, sometimes 

 from time to time at the instigation of the will. Again, 

 where nerve-cells occur, we find that excitements which 

 are transmitted to the central organ by a nerve-fibre 

 may there be imparted to other nerve-fibres. Thirdly, 

 we find that excitements which are transmitted to the 

 central organ by nerve-fibres there elicit a peculiar 

 process, which is called sensation and consciousness. 

 Fourthly and finally, the remarkable phenomenon, 

 mentioned above, of retardation, only occurs where 

 nerve-cells are present. The four following qualities, 

 which are entirely absent in nerv.e-fibres, must there- 

 fore be attributed to nerve-cells : 



(1) Excitement may arise in them independently, 

 i.e. ^vithout any visible external irritant. 



(2) They are 'able to transfer the excitement from 

 one fibre to another. 



