6g6 A MANUAL OF PHYSIOLOGY 



change their old end-organs for new ones ; pilo-motor fibres, for 

 instance, finding their way into the iris and becoming pupillo- 

 dilators. After excision of the superior cervical ganglion the 

 cervical sympathetic does not recover its function. Accordingly 

 the pre-ganglionic fibres cannot form direct functional con- 

 nection with the post-ganglionic fibres, but can become connected 

 with them only indirectly through the ganglion cells. Nor can 

 efferent post-ganglionic fibres achieve regenerative union with 

 a cerebro-spinal (somatic) motor nerve, although they can 

 themselves regenerate, as has been shown, e.g., in the case of the 

 vaso-constrictors of the limbs. On the other hand, union 

 easily takes place between pre-ganglionic fibres and efferent 

 somatic fibres, and vice versa. For example, the cervical 

 sympathetic can unite with the phrenic nerve, and cause con- 

 traction of the diaphragm, or with the recurrent laryngeal 

 nerve, and cause movement of the vocal cords, or with the 

 spinal accessory, and cause contraction of the sterno-mastoid 

 muscle. Conversely, the phrenic nerve, when united with the cer- 

 vical sympathetic, can, when stimulated, produce the usual effects 

 observed on exciting the latter nerve (Langley and Anderson) . 



Although the establishment of connection with the central 

 end of the cut nerve is necessary for complete regeneration, it 

 must not be supposed that no share whatever is taken in the 

 process by the peripheral stump. Even while it remains com- 

 pletely isolated from the central nervous system changes occur 

 which are often described as the third or final stage of degenera- 

 tion, but which are more correctly interpreted as forming a 

 stage in the regenerative cycle. Spindle-shaped cells or fibres 

 with elongated nuclei make their appearance, produced by the 

 proliferation of the nuclei of the primitive sheath already 

 described, and the increase of the protoplasm in which these 

 nuclei are embedded. These so-called axial strand fibres 

 or this fibrillated protoplasm may appear long before the 

 remains of the degenerated axis-cylinder and myelin sheath 

 have been completely removed. It is generally acknowledged 

 that in the adult they do not develop beyond this, so long 

 as the peripheral portion of the nerve remains completely isolated, 

 but neither do they disappear even after a very long interval. 

 When strict precautions against union with other nerve-trunks 

 were taken the radial nerve of an adult cat was found in this 

 resting-stage nearly a year and a half after division, and the same 

 was true after two years and a half in a nerve divided in a human 

 being. The fibres are incapable of being excited or of conducting 

 nerve impulses. The precise relation between these axial strand 

 fibres of the peripheral stump and the myelinated fibres found 



