THE CENTRAL NERVOUS SYSTEM 791 



the sensory supply of the corresponding cutaneous area. The com- 

 mon anatomical origin seems to carry with it a physiological corre- 

 lation, either because the irritation of the visceral fibres spreads in 

 the cord to the somatic afferent fibres which enter the corresponding 

 segments, or because of some action higher up in the cerebral centres, 

 the nature of which will be best considered along with the general 

 topic of the localization of sensory impressions (p. 982). 



Recurrent Sensibility. Although muscular contraction is 

 the most conspicuous event that follows stimulation of the 

 peripheral end of an anterior nerve-root, it is by no means the 

 only one. It is frequently observed, though not in all kinds of 

 animals, that here, too, pain is caused. That this pain is not 

 due to the muscular contraction is proved by the fact that it 

 can still be elicited when the nerve-trunk is divided between the 

 junction of the roots and the periphery. The real explanation 

 of the phenomenon is that certain fibres from the posterior 

 roots (' recurrent fibres/ see footnote on p. 693) bend up for 

 some distance into the anterior roots, and then turn round 

 again and pursue their course to their peripheral distribution 

 in the mixed nerve, or run on in the motor roots to supply the 

 sheath surrounding them (nervi nervorum), and even the mem- 

 branes of the spinal cord. 



The afferent impulses that enter the cord along the pos- 

 terior roots have the choice of many paths by which they may 

 reach the brain. The following are a few of the routes which 

 they may follow : 



(1) They may pass directly up through the postero-median column. 

 If they take this route, their course will be first interrupted by nerve- 

 cells in the gracile or cuneate nuclei in the medulla oblongata. 

 Thence they may find their way across the middle line by the arcuate 

 fibres of the upper or sensory decussation, and sweeping along the 

 fillet and the sensory path in the hinder part of the posterior limb 

 of the internal capsule, finally arrive at the cerebral cortex. Between 

 the gracile and cuneate nuclei and the cortex they pass through 

 nerve-cells in the optic thalamus. 



(2) They may pass up by the direct cerebellar tract and restiform 

 body to the grey matter of the cerebellar worm. If they take this 

 route their course will be interrupted very soon after their entrance 

 into the cord in the cells of Clarke's column. Since the superficial 

 grey matter of the vermis is connected by association fibres with 

 the dentate nucleus, and the dentate nucleus by the superior peduncle 

 with the opposite cerebral hemisphere, this is also a possible path 

 to the great brain. 



(3) They may reach the antero-lateral ascending tract of the same 

 side through its cells of origin in the spinal grey matter, and passing 

 through the medulla and pons to the superior peduncle of the. cere- 

 bellum, enter the grey matter of the superior worm. 



in the body. It embraces, in addition to the sympathetic, cranial auto- 

 nomic fibres in several of the cranial nerves and sacral autonomic fibres 

 in the nervi erigentes (see p. 883). 



