56 ELECTRO-PHYSIOLOGY CHAP. 



division rarely occurs so that both branches are equal in diameter ; 

 one twig is usually much finer than the other, and there is fre- 

 quently a marked disproportion, since a very thick axis-cylinder 

 may give off an excessively fine lateral branch. 



We shall have more to say later as regards the mode of 

 division of vertebrate nerves, especially under the interesting 

 relations which obtain in the electrical organ of the torpedo. 



The law of isolated conduction does not apply within the 

 central organs in the same sense as in the peripheral nerves 

 and their terminations. Here the conditions for irradiation of 

 excitation on all sides are obviously present, as appears more 

 particularly from the manifestations of strychnin tetanus, where 

 the stimulation of one or a few sensory nerve-fibres may, through 

 the spinal cord, throw nearly all the striated skeletal muscles into 

 active excitation. If, under normal conditions, the same localised 

 stimulus calls out one co-ordinated (reflex) movement only, con- 

 fined to one definite group of muscles, we may in a certain sense 

 speak of isolated conduction. But the reason why the excitation in 

 this case follows definite and invariably uniform paths, lies, not in 

 a sharply-defined anatomical connection of the nervous structures 

 involved (since these must on the .contrary be connected on all 

 sides, as regards conductivity, within the central organ), but in 

 certain special conditions, excitatory or conducting, along certain 

 " canalised " paths " or lines of discharge " in the gray matter. 



Wherever an excitable substance is endowed with highly- 

 developed conductivity, there is inevitably an -equal irradiation of 

 the excitatory process on all sides, so that it almost appears inevit- 

 able that each nerve-fibre, like a muscle-fibre, must conduct in both 

 directions. At the same time, the fact that every nerve-fibre is 

 naturally connected with an organ of excitation and a peripheral 

 organ, renders it impossible that any direction of conductivity, 

 other than from the former to the latter, should produce a recog- 

 nisable effect. Many efforts have, however, been made to obtain 

 a direct experimental proof of the matter. Such are more par- 

 ticularly the attempted union of the central end of sensory, and 

 peripheral end of motor, nerve-fibres that have been divided. 



Without going into the earlier and by no means unexcep- 

 tionable experiments of Bidder, Philipeaux, Vulpian, and others, 

 who endeavoured to unite the central stump of the sensory 

 ramus lingualis trigemini with the peripheral end of the hypo- 



