ANIMAL AND VEGETABLE 75 



certain processes which are useless to man ; I refer to 

 insulating processes. If their existence is disputed, I can 

 only reply that proof of their presence is to be found in 

 recognised works on Physiology. Let me make that clear. 

 Assume that we do not know anything about the nature of 

 the nerve impulse, and consider only the behaviour of 

 nerves under electrical stimulus or irritation. My authority 

 is Professor Rosenthal, who, in his Physiology of the 

 Muscles and Nerves, writes as follows : "If the main 

 stem of a nerve is irritated by electric 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 that the irritation yet 

 remains isolated in the separate fibres and is not imparted 

 to the neighbouring fibres, ^ihis statement holds good of 

 all peripheric nerves. Wherever it is possible to irritate 

 separate fibres the irritation is always confined to these fibres 

 and is not transmitted to those adjacent." * 



Now, the sciatic nerve is composed of a number of 

 bundles of nerve-fibres (some efferent, some sensory). If 

 each one was not separately insulated it would be im- 

 possible to irritate one fibre electrically without simul- 

 taneously irritating all the others. Not only is this so, but 

 each bundle is protected from inductive interference by a 

 lymph space directly under the perineurium and cor- 

 responding to the copper taping of telephone or telegraph 



* The italics are mine. 



