6o8 A MANUAL OF PHYSIOLOGY 



sheath, and develop into complete nerve-fibres, restoring 

 first sensation, and later on voluntary motion, to the para- 

 lyzed part. The process needs several months for its com- 

 pletion, even in warm-blooded animals. It takes place 

 under the influence of the nucleated portion of the neuron 

 (the cell-body), and never occurs if the peripheral and 

 central portions of the nerve are permanently separated by 

 a substance through which the new axis-cylinders cannot 

 grow or by a gap too wide for them to bridge over. When 

 the cut ends of the nerve are carefully sutured together, the 

 conditions for complete and speedy regeneration are rendered 

 more favourable, a fact which finds its application in the 

 surgical treatment of injured nerves. 



It is not as yet well understood how the regenerating fibres are 

 directed in their growth, so that they join their appropriate end- 

 organs without mistake. Doubtless the old nerve-sheaths serve to 

 some extent as guides by offering to the new axones a path of least 

 resistance. That this is the case is clearly shown by the results of 

 cross-suturing such nerves as the median and ulnar, *>., of uniting 

 the central end of the one with the peripheral end of the other. 

 Howell and Huber found that after this operation in the dog, both 

 co-ordinated voluntary motion and sensation returned in large measure 

 in the parts supplied by the nerves. Here the motor fibres of the 

 median nerve must of course have made connection with muscles 

 previously supplied by the ulnar, being guided to them along the 

 nerve-sheaths of the latter. That there is something more, however, 

 seems evident from the experiments of Langley on regeneration of the 

 cervical sympathetic m the cat after section below the superior 

 cervical ganglion. The nerve contains fibres of various functions 

 which reach it from the upper thoracic nerves. The anterior roots of 

 the first and third thoracic nerves supply the cervical sympathetic 

 mainly with fibres which end in the ganglion around cells that give 

 off dilator fibres for the pupil. The fibres connected with the cells 

 in the ganglion which send vaso-motor fibres to the vessels of the ear 

 are for the most part contained in the anterior roots of the second 

 and fifth thoracic nerves ; and the fibres connected with the cells that 

 give origin to the pilomotor fibres for the hairs of the face and neck 

 in the anterior roots of the fourth to the seventh. Stimulation of any 

 one of the upper thoracic roots accordingly causes a specific effect, 

 which, according to Langley, is in general the same after regeneration 

 as before section of the cervical sympathetic. We must assume, there- 

 fore, that each regenerating fibre seeks out either the ganglion cell 

 with which it was originally connected, or one belonging to the same 

 class. But under certain conditions these pre-ganglionic nerve-fibres 

 can form connections with nerve-cells of a different class, e.g., pupillo- 



