CHAP, i.] THE SPINAL CORD. 911 



with voluntary movements; the two can only be distinguished 

 from each other by a knowledge of the exciting cause. And it 

 seems unreasonable to suppose that the spinal cord should possess 

 two sets of mechanisms in all respects identical save that the one 

 is discharged by volitional impulses from the brain and the other 

 by afferent impulses from afferent nerves. 



We arc- led therefore to the conclusion that in a reflex action 

 t\\<> kinds of afferent impulses are concerned : the ordinary afferent 

 impulses which discharge the nervous mechanism within the cord 

 and so provoke the movement, and the afferent impulses which 

 connect that nervous mechanism with the muscles about to be 

 called into play, and which take part in the coordination of the 

 movement provoked. The nature of these latter afferent impulses 

 is at present obscure ; we know as yet little more than the fact of 

 their existence ; but if we admit, as we seem compelled to do, 

 that the character of a reflex action is determined by them as well 

 as by the afferent impulses which actually discharge the mecha- 

 nism, it seems possible that a fuller knowledge of these coordinating 

 afferent impulses may afford an adequate explanation of the fact 

 that when, as in the case of the frog in question, the usual set of 

 muscles cannot be employed by the nervous mechanism, recourse 

 is had to another set. 



We have avoided the introduction of the word ' consciousness ' 

 as unnecessarily complicating the question ; and it would be out 

 of place to discuss psychological problems here. We may remark 

 however that since we have no objective proofs of consciousness 

 outside ourselves, and only infer by analogy that such and such an 

 act is an outcome of consciousness on account of its likeness to 

 acts which are the outcome of our own consciousness, we conclude 

 that the brainless frog possesses no active consciousness like our 

 own, because absence of spontaneous movements seems to be 

 irreconcilable with the existence of an active consciousness whose 

 very essence is a series of changes. Consciousness as we recognize 

 it seems to be necessarily operating as, or to be indissolubly 

 associated with the presence of, an incessantly repeated internal 

 stimulus; and we cannot conceive of that stimulus failing to 

 excite mechanisms of movement which, as in the case of the 

 brainless frog, are confessedly present. We may however distin- 

 guish between an active continuous consciousness, such as we 

 usually understand by the term, and a passing or momentary 

 condition, which we may speak of as consciousness, but which is 

 wholly discontinuous from an antecedent or from a subsequent 

 similar momentary condition; and indeed we may suppose that 

 the complete consciousness of ourselves, and the similarly com- 

 plete consciousness which we infer to exist in many animals, has 

 been gradually evolved out of such a rudimentary consciousness. 

 We may, on this view, suppose that every nervous action of a 

 certain intensity or character is accompanied by some amount 



F. 58 



