124 THE PHYSIOLOGY OF NERVE 



normal appearance and again become functional. It might probably 

 be mentioned that this regeneration does not always lead to a reunion 

 of the same axis-cylinders; in fact, a union may be effected between the 

 central and distal stumps of two different motor nerves or their 

 branches. Quite similarly, a sensory nerve or a segment thereof may 

 be brought into functional connection with a motor nerve. Purpura, 

 for example, has obtained good functional results in cases of paralysis 

 of the face by joining the distal end of the facial nerve with the central 

 end of the spinal accessory. In animals the latter nerve has also 

 been united with the vagus nerve, this crossing enabling an ordinary 

 musculomotor nerve to produce an inhibition of the heart. 



Very important evidence favoring this centro-peripheral manner of 

 regeneration, has been presented by Harrison.^ It has been shown by 

 this investigator that the excision of the neural crest in the larvse of 

 amphibians, from which the cells of Schwann are derived, does not 

 hinder the development of the axis-cylinders but prevents their ac- 

 quiring medullary sheaths. It has also been demonstrated that nerve- 

 cells send out axis-cyhnders when immersed in a favorable nutritive 

 medium and that nerve-fibers are generated by pieces of cerebellum 

 and spinal ganglia when kept in a culture of clotted plasma. Many 

 of these axon processes attain a length of 0.5 mm. in the course of 

 48 hours. 



CHAPTER XII 

 THE PHENOMENA OF CONDUCTION IN NERVE 



Irritability and Conductivity. — Under normal conditions the wave 

 of excitation arises at one pole of the neuron and traverses it in a 

 definite direction, either afferently or efferently. Under experimental 

 conditions, on the other hand, it is possible to bring the stimulus to 

 bear upon it at almost any point, i.e., either upon its cell-body, its 

 axon or its end-organ. But the reaction remains the same in all cases^ 

 a motor effect resulting from the excitation of a motor nerve and a 

 sensation from that of a sensory nerve. The structural element pri- 

 marily concerned in this transmission of the wave of excitation is the 

 fibrillated axis-cylinder of the nerve-fiber and its ramifications inside 

 the cell-body. 



It is possible to differentiate between the irritability and conductivity of nerve 

 in the following manner: A muscle-nerve preparation (M) is placed horizontally 

 upon a glass plate, the nerve (A^) being drawn through a small glass chamber (D), 

 which in turn is connected with a Kipp apparatus (C). One pair of electrodes 

 are adjusted to the nerve inside this chamber (at A) and another pair outside of 



1 Harvey Lectures, New York, 1909, 199. 



