GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 79 



new structure, though possessing neither myelin nor axis-cylinder, i> found to 

 be capable of conduction and to have a low form of irritability, being ex- 

 citable to violent mechanical stimuli but not to induction currents. The power 

 of conduction appears to return before irritability, and may be observed first 

 at the end of the third week. Apparently sensation is recovered before the 

 power of making voluntary movements; this difference may well be due, not 

 to any essential difference between sensory and motor fibres, but to the tact 

 that extra time is required for the motor fibres to make connection with the 

 muscle. The embryonic fibre gradually gives place to the adult fibre, new 

 myelin being formed all along the fibre, and a new axis-cylinder growing down 

 from the old axis-cylinder. As the axis-cylinder grows down, the irritability 

 for induction shocks is recovered. Many months may be necessary for the 

 complete recovery of function. Langley ' reports that medullated fibres of 

 the sympathetic, if cut, regenerate and recover the power to function before 

 they regain a medullary sheath. Such experiments show the axis-cylinder 

 to be the true conducting medium, and that the medullary sheath has a sub- 

 ordinate function. 



The same is true of muscle as of nerve protoplasm, — the power of con- 

 duction ceases with the life of the cell-substance; thus, if the middle part of 

 a muscle-fibre be killed, by pressure, heat, or some chemical, the dead proto- 

 plasm acts as a block to prevent the state of activity which may be excited at 

 one end from being transmitted to the other, and the conduction power is only 

 recovered on the regeneration of the injured tissue. 



Isolated Conduction is the Rule. — (a) Conduction in Nerve-tmt/nks. — The 

 axis-cylinders of the many fibres which run side by side in a nerve-trunk are 

 separated from each other by the neurilemma, and in the case of the medullary 

 nerves by the myelin substance as well, so that there is not even contiguity, 

 much less continuity of nerve-substance. Thus the many fibres of a nerve- 

 trunk, some afferent and others efferent, though running side by side, conduct 

 independently of one another. For example, if the skin of the foot be pricked, 

 the excitation of its sense-organs is communicated to sensory nerve-fibres, and 

 is transmitted along them to the spinal cord, where the stimulus awakens cer- 

 tain groups of cells to activity; these cells in turn, by means of their branches, 

 the motor nerve-fibres, transmit the condition of excitation down to the mus- 

 cle-fibres of the legs, which, when stimulated, contract and withdraw the foot 

 from the offending irritant. The sensory and motor nerves concerned in this 

 reflex act run for a considerable part of their course in the same nerve-trunk, 

 but the sensory impulses have no direct effect on the motor nerve-fibres, and 

 the roundabout course which has been described is the only way by which 

 they can influence them. 



Isolated conduction by separate fibres and their branches holds good within 

 the central nervous system, as elsewhere, otherwise we could scarcely explain 

 the localization of sensations, or co-ordinated movements. 



The presence of a medullary sheath is not essential to isolated conduction, 

 1 Journal of Physiology, IS'.'T, vol. xxii. p. '2-A. 



