484 ROOTS Of SPINAL NERVES. [Boon in. 



sensory fibres. Lastly, if the posterior ganglion be excised, the 

 whole posterior root degenerates, as do also the sensory fibres of 

 the mixed nerve-trunk. Putting all these facts together, it would 

 seem that the growth of the motor and sensory fibres takes place 

 in opposite directions, and starts from different nutritive or 

 'trophic' centres. The sensory fibres grow away from the ganglion 

 either towards the periphery, or towards the spinal cord. The 

 motor fibres grow outwards from the spinal cord towards the 

 periphery. This difference in their mode of nutrition is frequently 

 of great help in investigating the relative distribution of motor 

 and sensory fibres. When a posterior root is cut beyond the 

 ganglion, or the ganglion excised, all the sensory nerves degenerate, 

 and the sensory fibres, by their altered condition, can readily be 

 traced in the mixed nerve-branches. Conversely, when the an- 

 terior roots are cut, the motor fibres alone degenerate, and can be 

 similarly diagnosed in a mixed nerve-tract. When the anterior 

 root is divided some few fibres in it do not, like the rest, degene- 

 rate, and when the posterior root is divided, a few fibres in the 

 anterior root are seen to degenerate like those of the posterior 

 root ; these appear to be the fibres which give to the anterior root 

 its "recurrent sensibility." By the same means in a mixed nerve 

 like the vagus, the fibres which spring from the real vagus root 

 may be distinguished from those proceeding from the spinal 

 accessory, by section of the vagus and spinal accessory roots 

 respectively ; and in the mixed vago-sympathetic trunk, met with 

 in many animals, the vagus fibres may be distinguished from the 

 sympathetic, since, after a section of the mixed trunk, the former 

 degenerate from above downwards, whereas the latter degenerate 

 in an upward direction from the inferior cervical ganglion below to 

 the superior cervical ganglion above; for the ganglia of the 

 sympathetic behave in this respect like the spinal ganglia of the 

 posterior roots. This method of diagnosis is often spoken of as 

 the Wallerian method, after A. Waller, to whom we are indebted 

 for the discovery of most of these facts. 



In the cranial nerves the motor and sensory tracts are far less 

 mixed than in the spinal nerves. The olfactory, optic and acoustic 

 nerves are purely sensory nerves. The fifth, glosso-pharyngeal 

 and vagus are mixed nerves ; and it is stated that in the dog the 

 afferent and efferent fibres of the vagus are gathered into two 

 bundles so distinct that they may be separated by the knife, 

 the afferent bundle lying to the outside of the efferent bundle. 

 The facial and hypoglossal are for the most part motor (efferent) 

 nerves, but contain sensory (afferent) fibres. The third, fourth, 

 sixth and spinal accessory are exclusively motor (efferent) nerves. 

 These statements refer to what are commonly looked upon 

 as the trunks of the respective nerves. More exactly speaking, 

 the sensory fibres of the facial come from the fifth, pneumo- 

 gastric and glosso-pharyngeal nerves, so that the facial proper is in 



