viii CONDUCTIVITY AND EXCITABILITY OF NERVE 69 



of the centrifugal (motor) nerve ; this is the " reflex time " proper 

 (Exner's " reduced reflex period ") ; (3) the time required by 

 the excitation to traverse the motor nerve and evoke a contrac- 

 tion in the muscle. 



The rate of conductivity usually accepted for frog's nerve, i.e. 

 27 m. per sec., is by no means an invariable figure. Helmholtz 

 indeed assumes that the excitation is transmitted at constant 

 rapidity, but this is neither certain nor even probable. These 

 figures must therefore stand for an upper limit of the reflex period, 

 and in longer nerves than those employed there would presumably 

 be a lower rate of transmission. Even with these reservations, 

 however, Helmholtz's facts are incontestable. The excitatory pro- 

 cess undergoes a considerable retardation during its passage 

 through the spinal cord from sensory to motor fibres. This delay 

 must be attributed to the structure ' of the nerve-cells, in virtue of 

 which they are distinct from their processes, the nerve-fibres. The 

 difference of constitution is best expressed by saying that the 

 central nervous organ presents greater resistance to the transmis- 

 sion of excitation than the sensory or motor peripheral paths. 



Like rapidity of conduction in peripheral nerve, only in a 

 much higher degree, the reflex time, as the expression of the rate 

 at which excitation is transmitted within the central organs, is 

 conditioned by several factors, and is itself very variable. 



The length of tract within the central organ, or, as it may 

 perhaps be expressed, the number of ganglion - cells to be 

 traversed, is of great importance. One fact must be mentioned by 

 which central is distinguished from peripheral conductivity, 

 and is rendered very complex. Each central organ consists 

 of course of a multitude of nerve-cells, with centripetal and centri- 

 fugal fibres. If the law of isolated conduction obtained strictly 

 within the centres also, so that each conducting path was isolated 

 as in the peripheral nerves, each impulse from an afferent nerve- 

 fibre could have but one definite localised effect, that would never 

 vary under any circumstances. If, on the other hand, we recog- 

 nise that the connection of different fibres by ganglion -cells 

 (which finally establish a closer or more remote relation between 

 all departments of the central organ) admits of conduction in 

 all directions with equal facility, then impulses arriving at any 

 part of the central organ would radiate diffusely without pro- 

 ducing any definite and localised action. Neither the one nor 



