294 PHYSIOLOGY CHAP. 



Demonstration of this phenomenon, which Longet termed recurrent 

 sensibility, was given in the following experiments : 



(a) If the ventral root be cut, and the two stumps are then 

 stimulated, sensory effects are obtained from the peripheral end 

 only, while excitation of the central stump produces no effect. 



(b) If a dorsal root be cut, the sensibility of the corresponding 

 ventral root disappears entirely, whether this be cut or not. 



Claude Bernard discovered that in order to obtain a good 

 demonstration in the dog of the sensibility of the ventral roots, it 

 is necessary to wait about an hour after exposing the cord. If the 

 sensibility of the roots is tested immediately after the vertebral 

 canal has been opened, it is always found that the dorsal roots 

 alone respond to stimulation. This fact is incontestable, but the 

 explanation given by Bernard appears to us incorrect. He assumes 

 that the recurrent sensory fibres of the ventral roots become 

 insensitive owing to the shock of the operation, and recover their 

 sensibility with rest. But under normal conditions not only the 

 ventral roots through which the recurrent fibres pass, but also the 

 meninges of the cord to which they are distributed, are insensitive 

 like all serous membranes, and they become sensible to pain only 

 when inflammation, due to exposure to the air and other influences, 

 sets in. Hence we may conclude that the fibres that run back 

 from the dorsal to the ventral roots to be distributed to the 

 meninges belong to that category of centripetal nerve-fibres that 

 abound in all visceral organs, and are normally devoid of con- 

 scious sensibility ; excitation of these only passes the threshold 

 of consciousness to arouse sensations of pain under conditions of 

 irritation or inflammatory reaction. 



Bernard further demonstrated that division of the mixed nerve 

 trunk at a certain distance from the union of the two spinal roots 

 abolishes the sensibility of the ventral root, in the same way as 

 after division of the dorsal root. This fact proves that the point at 

 which the recurrent fibres turn centripetally is not at the junction 

 of the roots, but in the nerve plexuses or more peripherally. 



Bernard further believed that he had demonstrated that the 

 sensibility of each ventral root was dependent solely on the corre- 

 sponding dorsal root, and not on other adjacent sensory roots, but 

 the subsequent researches of Arloing and Tripier show that recur- 

 rent fibres may pass from a sensory to other sensory roots. 



The existence of recurrent centripetal fibres in the ventral 

 roots makes it highly probable that centrifugal fibres may emerge 

 from the ventral roots to run back in the dorsal roots to innervate 

 the muscle cells that occur in the interior or on the surface of 

 the cord (vasoinotor fibres). Vulpian, on exciting the peripheral 

 stump of a ventral root, was unable to detect any visible alteration 

 of circulatory conditions at the surface of the cord, but this negative 

 result is possibly due to the fact that the vasomotor nerves of the 



