774 NERVE 



regeneration of the cervical sympathetic in 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 pilo-motor 

 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, therefore, that each 

 regenerating fibre seeks out either the ganglion cell with which it 

 was originally connected, or one belonging to the same class. No 

 mere mechanical guidance of the growing axons by the old neuri- 

 lemmas will suffice to explain this selective growth. It is necessary 

 to postulate, in addition, an attraction of a chemical or physico- 

 chemical nature (chemiotaxis), dependent upon a specific relation 

 between the new axons and the scaffolding of the peripheral stump 

 or the ganglion cells. But it is not possible at present to form any 

 very precise conception of the properties on which the chemiotactic 

 phenomena depend. And the specificity is not an absolute one. 

 Under certain conditions these pre-ganglionic nerve-fibres (that is 

 to say, nerve-fibres running from the spinal cord to end around the 

 sympathetic ganglion cells) can form connections with nerve-cells 

 of a different class e.g., pupillo-dilators with cells whose axons 

 end in the erector muscles of the hairs. Further, after section of 

 the sympathetic above the superior cervical ganglion, the post- 

 ganglionic nerve-fibres (i.e., the fibres coming off from the cells of 

 the ganglion) may also, if the opportunity be favourable during 

 regeneration, exchange 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 connection 

 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 re- 

 generate, 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 



