ISOLATED TRANSMISSION. 117 



this it may be inferred that a resistance to the trans- 

 mission exists within the nerve, and that this gradually 

 retards the rate of propagation. Such a resistance to 

 transmission is also probable on certain other grounds, 

 to which subject we shall presently revert. 



4. If the main stem of a nerve is irritated by elec- 

 tric shocks, all the fibres are invariably simultaneously 

 irritated. On tracing the sciatic nerve to its point of 

 escape from the vertebral column, it appears that it is 

 there composed of four distinct branches, the so-called 

 roots of the sciatic plexus. These rootlets may be 

 separately irritated, and when this is done contractions 

 result, which do not, however, affect the whole leg but 

 only separate muscles, and different muscles according 

 to which of the roots is irritated. Now as the fibres 

 contained in the root afterward coalesce in the sciatic 

 nerve within a membrane, it follows from the experi- 

 ment just described that the irritation yet remains 

 isolated in the separate fibres and is not imparted to 

 the neighbouring fibres. This statement holds good of 

 all peripheric nerves. Wherever it is possible to irri- 

 tate separate fibres the irritation is always confined to 

 these fibres and is not transmitted to those adja- 

 cent. We shall afterwards find that such transmis- 

 sions from one fibre to another occur within the cen- 

 tral organs of the nervous system.' But in these cases 

 it can be shown with great probability that the fibres 

 not only lie side by side, but that they are in some 

 way interconnected by their processes. In peripheric 

 nerve-fibres the irritation always remains isolated. 

 Their action is like that of electric wires enclosed in 

 insulating sheaths. One of these nerves may indeed 

 be compared to a bundle of telegraph wires, which are 



