45 6 



NER VE. 



advantage of. 



Transmission in both directions. — Another aspect of this funda- 

 mental fact of propagation is that known as transmission in both 

 directions. If a nerve fibre is stimulated in the course of its continuity, 

 the excitatory state is transmitted along both its peripheral and its 

 central portions. The property possessed by the nerve is thus one of 

 conductivity in both directions. 



It is clear that it is not easy to secure proofs of such doubly 

 directed propagation, since the index for either the efferent or the 

 afferent nerve is afforded by the response of a structure (muscle or 

 central nervous system) situated at one end only. In order to secure 

 a double terminus, each capable of responding on the arrival of the 

 excitatory state in precisely the same way, the division of efferent axis 

 cylinders in the neighbourhood of their terminations has been taken 

 In the sartorius muscle of the frog a suitable nerve 



termination may 

 be found, since 

 the axis cylinders 

 divide into two 

 main groups, each 

 of which ascends 

 some distance to- 

 wards the upper 

 iliac end between 

 the parallel mus- 

 cular fibres. It is 

 possible to divide 

 this end of the 

 muscle into two 

 halves for some 

 little distance ; 

 the stimulation of 

 either tip, where 

 there are no 

 nerves, evokes a 

 response in that 

 strip only, the 

 excitation of the 

 strip where the 

 nerves are present 

 evokes a response on both sides. The gracilis muscle may be so cut 

 that different regions shall have independent muscle fibres, but retain 

 a nerve supply from the divisions of the same axis cylinders (Fig. 251). 

 A similar double response of its separated regions is evoked when any 

 one part with its contained nerve fibres is stimulated. 1 The electrical 

 organ of Malapterurus is a notable instance of an enormous nerve 

 supply attained by the divisions of a single axis cylinder. It can be 

 detached from the fish, and the nerve with its single axis cylinder can 

 be divided where it courses upon the inner surface of the organ, and 

 excited at either its central or its peripheral end. Further, any single 

 branch may be cut off from its peripheral connections, and similarly 

 excited. The response of the electrical organ is a very distinct one, 

 and is evoked in every part, whether a peripheral twig or the main 



1 Kiihne, Ztschr.f. Biol., Bd. xxii. S. 305. 



Fig. 251. — A, Sartorius split at iliac end ; excitation at x 1 or x. 2 

 evokes contraction of excited half only; excitation at x 3 or 

 x 4 evokes contraction in both halves. B, Gracilis with 

 peninsula of muscle cut, so that the continuity of its fibres is 

 interrupted. Excitation of nerve in muscle peninsula (x x ) 

 evokes contraction in all parts of the muscle. C, Three por- 

 tions of Malapterurus electrical organ, connected by single 

 nerve fibre. Excitation at either x x or x„ evokes an electrical 

 response in all three portions. 



