THE CENTRAL NERVOUS SYSTEM 699 



any part of the skin ; and Sherrington has found that 

 the cutaneous areas of distribution of consecutive nerve- 

 roots are not perfectly independent, but to some extent 

 overlap. Stimulation of the peripheral end of the divided 

 posterior root has no effect. Stimulation of the central end 

 gives rise, if the animal be conscious, to evidences of pain, 

 and other signs of the passage of afferent impulses, e.g., a 

 rise in blood-pressure. The latter may also be observed 

 when the animal is anaesthetized. 



The posterior roots contain sensory fibres not only for the skin, 

 but also for the deeper structures and the viscera. Recent clinical 

 observations have thrown much light upon the distribution of the 

 visceral fibres and their relation to the cutaneous sensory nerves. 

 It has long been known that in disease of an internal organ the pain 

 is often referred to some superficial part. It has now been demon- 

 strated that each organ is related to a definite region of the skin. 

 In disease of the organ the excitability for impressions of touch or 

 temperature is increased in this area, and the reflexes elicited by 

 stimulation are exaggerated (Head, Dana). 



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

 spinal segment of the sympathetic sensory fibres of any viscus and 

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

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

 lation, the nature of which will be best considered along with the 

 general topic of the localization of sensory impressions (Chap. XIII.). 



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 seems to be that 

 certain fibres from the posterior roots (' recurrent fibres,' 

 see footnote on p. 607) bend up for some distance into the 

 anterior roots, and then turn around again and pursue their 

 course to their peripheral distribution in the mixed nerve, 

 or run on in the motor roots to supply the sheath surround- 

 ing them (nervi nervorum), and even the membranes of the 

 spinal cord. 



The afferent impulses that enter the cord along the pcs- 



