NERVE-FIBRES AND NERVE-CELLS. 107 



almost every cell a process may be distinguished which 

 is altogether distinct in appearance from the rest. The 

 protoplasmic processes become gradually finer and sepa- 

 rate into more parts, and the processes of neighbouring 

 cells are partly connected together. But the one pro- 

 cess which is distinguishable from the rest passes along 

 for a certain distance as a cylindrical cord, and then, 

 suddenly becoming thicker, it encases itself in a me- 

 dullary sheath, and in appearance entirely resembles 

 the medullary fibres of the peripheric system. It is 

 extremely probable, although it is hard to prove it with 

 certainty, that a fibre of this sort passing out of the 

 dorsal marrow is directly transformed into a peripheric 

 "nerve-fibre, while the protoplasmic processes continu- 

 ing on their course within the central organ serve to 

 connect the ganglion-cells. 



The nerve-system, the main parts of which we have 

 thus roughly examined, effects the motions and sensa- 

 tions of the body. These qualities belong, however, 

 mainly to the central parts, in which ganglion-cells 

 occur. The peripheric nerve-fibres act merely as con- 

 ducting or transmitting apparatus to or from the 

 central organs. Before examining the peculiar action 

 of the central nervous system, it is desirable to devote 

 some attention to this conducting apparatus and to dis- 

 cover its nature. 



2. On exposing one of the peripheric nerves of a 

 living animal and allowing irritants to act upon this, 

 in the way which was described in the case of muscles, 

 two effects are usually observable. The animal suffers 

 pain, which it expresses by violent motion or cries, and, 

 at the same time, individual muscles contract. On 

 tracing the irritated nerve to the periphery, it will be 



