198 PHYSIOLOGY CHAP. 



appeared, which is a proof of centrifugal conduction in the sensory 



fibres. 



Many other attempts have been made to demonstrate the 

 possibility of reversal of the normal passage of excitation along a 

 nerve. Schwann divided the sciatic of a frog, and allowed the 

 two ends to unite. He then stimulated the sensory roots of the 

 nerve, and saw that its excitation produced no contraction in the 

 muscles of the limb. From this he concluded against the theory 

 of conduction in a double direction, since it seemed to him im- 

 probable that each afferent or efferent fibre of the two stumps 

 should be able to unite with a fibre of its own kind. But the 

 fact that normal sensibility and motility is recovered after nerve 

 section shows that what Schwann thought so impossible really 

 does take place. His experiments, which Steinbrlick confirmed in 

 1838, do not therefore overthrow the theory of conduction in 

 both directions. 



Bidder (1841) attempted to connect the peripheral end of the 

 hypoglossal (motor nerve) with the central end of the lingual 

 (sensory nerve), but he only managed to unite trunks of the same 

 kind, as in Schwann's experiments. Union of heteronomous 

 stumps was, however, obtained by the subsequent experiments 

 of Gluge and Thiernesse (1859), Philipeaux and Vulpian (1860), 

 Kosenthal (1864), and Bidder himself (1865). It was found that 

 when the two nerves above mentioned had united, stimulation of 

 the lingual produced movements of the tongue, and stimulation 

 of the hypoglossal (united to the central end of the lingual) elicited 

 signs of pain. 



These results seemed to be positive evidence for conduction in 

 both directions; subsequent researches, however, proved them 

 capable of a different interpretation. The symptoms of pain when 

 the hypoglossal was stimulated can, according to Arloing and 

 Tripier, be interpreted as a phenomenon of recurrent sensibility in 

 the stump of the hypoglossal, and the movements of the tongue 

 on stimulating the lingual may, according to Vulpian's last work, 

 depend on excitation of the fibres of the chorda tympani, which is 

 an efferent nerve. If, on the other hand, the hypoglossal on one 

 side be cut so that it degenerates completely, and the peripheral 

 stump of the freshly divided lingual nerve is then excited, a slow 

 contraction of the tongue follows, which is due to the chorda 

 tympani and is accompanied by vascular dilatation. The mechanism 

 of this phenomenon is very obscure, since the chorda tympani has 

 no direct anatomical connection with the tongue muscles, and 

 produces no motor effect under normal conditions, i.e. when the 

 hypoglossal is uninjured. So that none of these experiments are 

 of any account for the question of double conductivity in nerve. 



Nor can any greater value be assigned to the experiments which 

 Paul Bert carried out on rats by suturing the tip of the tail to the 



