790 A MANUAL OF PHYSIOLOGY 



What sort of impulses do the various tracts of the spinal 

 cord conduct ? For the dorsal or posterior roots this question 

 was first fully answered by Magendie ; for the ventral or 

 anterior roots, although with a certain degree of ambiguity, 

 by Sir Charles Bell. Bell observed that when, in an animal 

 just killed, he mechanically stimulated the anterior roots, 

 muscular contractions were obtained at each touch of the 

 forceps. He concluded that the anterior roots are motor and 

 sensory, while the posterior roots are ' vegetative ' i,e., con- 

 nected with the functions of the viscera, the so-called ' vegeta- 

 t ive ' organs. But although he is often credited with the 

 discovery of the functions of the posterior roots as well, he 

 was not the first to make the decisive experiment necessary 

 to show that they are the conductors of sensory impulses. It 

 was after Magendie's discovery that only a portion of the nerves 

 are sensitive, and that there are nerves ' which are like tendons, 

 aponeurose?, or cartilages in insensibility ' that Bell formu- 

 lated the law that the anterior roots are purely motor, the 

 posterior purely sensory. This law, often termed Bell's Law, 

 is more correctly denominated the Bell-Magendie Law. 



When the posterior roots are divided, loss of sensation occurs 

 in the region to which they are distributed. If only one root 

 is cut, the loss of sensation is never complete in any part of the 

 skin ; and Sherrington has found that the cutaneous areas of 

 distribution of consecutive nerve-roots are not perfectly inde- 

 pendent, but to some extent overlap. Stimulation of the peri- 

 pheral end of the divided posterior root has no effect. Stimula- 

 tion 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. 



Referred Pain. The posterior roots contain sensory fibres not only 

 for the skin, but also for the deeper structures and the viscera. The 

 afferent fibres reach the viscera by the sympathetic, the vagus, and 

 the pelvic nerves or nervi erigentes. 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 demonstrated 

 that each organ is related to a more or less 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 



* Langley uses the term autonomic nervous system to include the 

 nerv* supply of. heart muscle, all unstriated muscle, and all gland cells 



