ON THE MAMMALIAN NERVOUS SYSTEM. 411 



that involving the activity of the corpuscles. If, then, on exciting a mixed nerve it 

 be found that excitatory processes are pi esent in a distant portion of the cord under 

 investigation, these may be due to - 



(1.) The propagation of these processes from the point of stimulation along direct 

 and continuous nerve fibres which pass into the cord and on through the investigated 

 region ; 



(2.) To the reflex discharge of interposed cells in the cord, brought about by the 

 arrival of excitatory processes which, having travelled up the nerves from the point 

 of stimulation, have entered upon paths ending in cells. These cells being thus 

 awakened, themselves start nerve impulses which travel on up the cord, and, reaching 

 the investigated area, produce the observed electrical effects ; 



(3.) To the mixture of effects produced by impulses both in direct paths and the 

 indirect ones just described. 



It must, therefore, be borne in mind throughout the present research that these 

 different causes may be operating in the production of the electrical effects observed ; 

 hence, one of our first experimental enquiries was to endeavour as far as possible to 

 discriminate between the effects referable to the direct fibres actually excited, and 

 those which were related but indirectly with these fibres, being primarily associated 

 with channels connecting the area of investigation with corpuscular mechanisms in the 

 cord. 



This we have attempted to do by employing such a low intensity of stimulation as 

 should in any particular degree of narcosis be sufficient to evoke nerve impulses, as 

 evidenced by electi ical changes, and, at the same time, not so intense as to arouse 

 any sign of reflex movement. The weak impulses thus generated in the excited nerve 

 presumably pass for the most part, if not entirely, along direct nerve fibres in the 

 cord which are not in relation with nerve cells before arriving at the region observed 

 (mid-dorsal) ; the evidence to be detailed later strengthens this presumption. 



The subject matter now dealt with really comprises the whole question of the rela 

 tion of the bulbo-spinal system to the lumbar nerves. It is obvious that this relation 

 ship is one which may be discussed under three heads according to the particular part 

 of the nerve tract under observation. 



First, the characters of the nerve impulses which pass along the channels of the 

 cord after entering it by its nerve roots ; 



Second, the characters of the nerve impulses which, aroused by excitation of the 

 channels of the cord and passing out into its nerve roots, descend along fibres of 

 the mixed nerve ; 



Third, the characters of the nerve impulses which enter and leave the cord by 

 virtue of its so-called central structures and their various connections. 



Of these three groups, the first alone will be treated of in the present chapter, the 

 second and third being reserved for Chapters X. and XI. respectively. 



3 G 2 



