GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 85 



fibre be artificially irritated at either end, the exciting change runs the length 

 of the fibre, regardless of the direction, and stimulates every pari of it to con- 

 traction. 



(6) In Nerves. — In the cases of nerves where excitation is accompanied by 

 no visible manifestation of activity, a definite answer to the question i> not so 

 readily obtained. As long as a nerve is within the normal body, the activity 

 of the nerve-fibre can only be estimated from the response of the cell which the 

 nerve-fibre excites, and there is such an organ onlyal one extremity of the fibre. 



Paul Bert made a well-known experiment, in which he tried t<> reverse a 

 sensorv nerve in the living animal. He succeeded in bringing about union 

 of the end of the tail of a rat with the tissues of the back, and found, when 

 the union was complete, alter the tail was cut off at its base, it was still capa- 

 ble of giving sensations of pain. The experiment tailed to throw light on 

 the problem, however, for we now know that the peripheral part of the cut 

 nerve dies, and the conduction power manifested in this case was dependent 

 on new axis-cylinders which had grown down from the central nerve-stump 

 (see p. 79). 



Efforts have been made to elucidate the problem by attempting to unite 

 the central part of a cut sensory nerve with the peripheral part of a divided 

 motor nerve, and observing, after the healing was complete, whether excita- 

 tion of the sensorv nerve caused movements in the pari supplied by tie' 

 motor nerve. Most of these experiments have given doubtful results, but 

 lately Budgett and Green ' have succeeded where others have failed, ami 

 have made cut sensory fibres grow down the degenerated trunk of a motor 

 nerve, and connect with muscle-fibres, so that the muscle contracted when 

 the peripheral end of the sensory fibres was stimulated. The impulse wenl 

 up the old sensorv fibres, and then down the newly developed fibres in the 

 old motor trunk. Their method was to cut the left pneumogastric nerve 

 between the ganglion and the cranium, and to suture its peripheral cut end 

 to the peripheral cut end of the hypoglossal. All the fibres of the hypoglossal 

 and the efferent fibres of the pneumogastric must have degenerated, because 

 these fibres were separated from the bodies of the cells of' which they were 

 branches. The sensory fibres of the pneumogastric, on the other hand, be- 

 cause still in connection with the nerve-cells of the ganglion, continued to 

 live, and tin; part connected with the peripheral -tump of the cut hypoglossal 

 grew down this nerve and came into relation with the muscles of the tongue. 



Two or three months after the operation the left pneumogastric was divided 

 just above the thorax, and the combined vago-hypoglossal nerve, together 

 with tin; tongue, was excised. When the peripheral end of the pneumogastric 

 was excited the muscles of the tongue were seen t<> contract. Mechanical as 

 well as electrical stimuli were effective, and there would seem to be no escape 

 from the conclusion thai tin- sensory fibres of the pneumogastric had con- 

 ducted the impulse ecntripctally as far as the ganglion, and t hen centrifugally 

 down to the muscle of the tongue. 



1 Budgett and (ircen : American Journal of Physiology, 1S99, iii. p. 1 15. 



