196 PHYSIOLOGY CHAP. 



fresh state, or when dehydrated only, which returns when con- 

 ductivity is re-established. 



One of the most important facts, which may rank as a funda- 

 mental law of nerve conduction, is that each fibre of a nerve 

 conducts the excitatory impulse from the periphery to the centre, 

 or from the centre to its terminal ramifications, without spread of 

 the excitation by contact to the neighbouring fibres. In the case 

 of a mixed nerve the motor fibres can be excited along their 

 course without simultaneously producing sensations, or the sensory 

 fibres without simultaneous production of movements. The most 

 convincing proof of isolated conduction of the active state in 

 individual fibres is afforded by the delicacy of localisation, both of 

 movements and, still more, of sensations. It is possible to stimulate 

 the small bundle of fibres that form the motor roots of the 

 sciatic separately so as to produce localised contractions in the 

 individual muscles or portions of muscles which they innervate, 

 without diffusion of the impulse to the whole group of muscles 

 that are thrown into action by stimulating the trunk of the sciatic. 

 The excessively delicate localisation of tactile sensations, the 

 sharpness of outlines and shading of colours in visual images, 

 would be quite impossible if each fibre of a peripheral or optic 

 nerve were not an isolated conductor. 



This localisation of movements and sensations, with which we 

 are all familiar, has so far received no mechanical explanation. It 

 has been thought on good evidence that the sheaths, and particularly 

 the myelin sheath, are mainly responsible for the complete insula- 

 tion of the axis-cylinder ; but the fact that this insulation holds 

 good for the non-medullated nerve-fibres as well leads one to 

 conjecture that it is a property inherent in the axis -cylinder, 

 though we are ignorant of the cause to which it is due. That 

 insulated conduction does not depend on the medullary sheath 

 is further proved by the fact established by Ducceschi, that when 

 the frog's sciatic is so compressed as to rupture the sheath without 

 blocking the conductivity of the nerve, isolated contraction of the 

 separate muscles of the foot can be obtained by stimulating single 

 branches of the lumbro-sacral plexus. 



The new theory of the minute structure of the nervous system, 

 according to which the axis-cylinder and the dendrites are con- 

 sidered not as elementary nerve-fibres but as bundles of separate 

 fibrils forming an elementary network, naturally raises the question 

 whether the law of insulated conduction is applicable to the pro- 

 cesses (dendrites and axons) of the ganglion cells as a whole, or 

 to the individual fibrillary elements of which these seem to 

 consist. It must be confessed that science is not yet ready to 

 solve this problem, which needs a more complete knowledge of 

 their anatomical relations. We can only say that many ramifica- 

 tions of nerve-fibres are merely dissociations of distinct fibrils 



