iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 259 



of certain analogies, proposed the theory that every excitation of 

 living matter (conceived as a system of semipermeable membranes) 

 induces change in the concentration of the ions, and that the 

 resulting concentration currents set up conduction in the nerve. 

 In this way, as pointed out by Boruttau (1904), it is possible to 

 reconcile the two opposite theories, physical and chemical, by 

 assuming that conduction in the nerve depends upon the electrical 

 currents produced by chemical metabolism. 



This theory, which Verworn has also (1906) accepted, presents 

 the further advantage of not being confined to nervous tissue, since 

 it is applicable to all other tissues of the body. 



X. We have seen that the function of the nerve-fibre is to 

 conduct excitation. Under normal conditions the excitatory 

 impulse never arises in the fibres, hence they are not capable of 

 transforming or reinforcing the impulses transmitted, either from 

 the periphery (centripetal or afferent nerves), or from the centres 

 (centrifugal or efferent nerves). The excitability of nerve-fibres 

 is rather a condition of their conductivity than an autonomous 

 property. But when the centripetal impulse has reached the 

 central grey matter the afferent impulse is transformed into an 

 efferent impulse. This transformation consists not in a simple 

 reversal of direction of the impulse, but in a discharge of fresh 

 energy, in which there is often a marked disproportion between 

 the afferent and the resulting efferent impulses. When, e.g., a 

 foreign body comes in contact with the glottis, a loud fit of 

 coughing is reflexly excited. This indicates that the stimulation 

 of a few sensory fibres is able in the centres to produce spread of 

 excitation to the motor fibres of all the expiratory muscles. There 

 is thus in the centres an explosion of fresh energy, comparable to 

 that discharged in the muscle when the excitation reaches the 

 end-plates along the motor nerves. 



The transformations which the afferent impulses undergo in 

 the centre can also be expressed as a diminution or inhibition of 

 pre-existing activities. The foreign body which provokes reflex 

 coughing when it touches the glottis does not merely throw the 

 motor centres of the expiratory muscles into activity, but it 

 simultaneously inhibits the activity of the inspiratory muscles. 

 Every co-ordinated reflex presents this double action of afferent 

 impulses on the central organ. The afferent impulses are also 

 capable of setting up processes which lead to the facilitation 

 (Bahnung) l of other reflex acts. 



While conductivity is the fundamental physiological function 

 of the peripheral nerve-fibres since we have no proof that these 

 modify impulses during conduction, excitability is the funda- 

 mental function of the nerve-centres, so that a weak impulse 



1 Bahnung has been variously rendered in English as facilitation, reinforcement, 

 canalisation, augmentation. TK. 



