864 THE CENTRAL NERVOUS SYSTEM 



much light upon the distribution of the visceral fibres and their rela- 

 tion to the cutaneous sensory nerves. It has long been known that 

 in disease of an internal organ the pain is often referred to some super- 

 ficial part. It has now been demonstrated that each organ is related 

 to a more or lees definite region, or more than one region, of the skin. 

 In disease of the organ there is in this area increased excitability 

 (hyperalgesia) or tenderness to slight mechanical stimuli, and often 

 also increased excitability for heat or cold, and the reflexes elicited by 

 stimulation are exaggerated (Head, Dana). 



The bond of connection appears to be the origin from the same spinal 

 segments of the autonomic* sensory fibres of any viscus and the sensory 

 supply of the corresponding cutaneous area. The common anatomical 

 origin seems to carry with it a physiological correlation, 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 (Chapter XVIII.) . 



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 con- 

 traction 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. 771) bend up for some distance into the anterior roots, and then 

 turn round again and pursue their course to their peripheral dis- 

 tribution 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 posterior 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 Jie 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 



* Langley uses the term autonomic nervous system to include the nerve 

 supply of heart muscle, all unstriated muscle, and all gland cells in the body. 

 It embraces, in addition to the sympathetic, cranial autonomic fibres in 

 several of the cranial nerves and sacral airtonomic fibres in the nervi erigentes 

 (see Chapter XVII.). 



