572 THE SPINAL CORD. 



and a passing and momentary condition, which we may speak of as con- 

 sciousness, 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 complete con- 

 sciousness 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 ac- 

 companied by some amount of consciousness, which we may, in a way, com- 

 pare to the light emitted when a combustion, previously giving rise to 

 invisible heat, waxes fiercer. We may thus infer that when the brainless 

 frog is stirred by some stimulus to a reflex act, the spinal cord is lit up by a 

 momentary flash of consciousness coming out of darkness and dying away 

 into darkness again ; and we may perhaps further infer that such a passing 

 consciousness is the better developed the larger the portion of the cord in- 

 volved in the reflex act and the more complex the movement. But such a 

 momentary flash, even if we admit its existence, is something very different 

 from consciousness as ordinarily understood, is far removed from intelli- 

 gence, and cannot be appealed to as explaining the " choice " spoken of 

 above. 



503. Lastly, the characters of a reflex movement are, as we need 

 hardly say, dependent on the intrinsic condition of the cord. The action 

 of strychnine just alluded to is an instance of an apparent augmentation 

 of reflex action best explained by supposing that the resistances in the cord 

 are lessened. There are probably, however, cases in which the explosive 

 energy of the nervous substance is positively increased above the normal. 

 Conversely, by various influences of a depressing character, as by various 

 anaesthetics or other poisons, reflex action may be lessened or prevented ; 

 and this again may arise either from an increase of resistance or from a 

 diminution in the actual discharge of energy. So also various diseases may 

 so affect the spinal cord as to produce, on the one hand, increased reflex ex- 

 citability, so that a mere touch may produce a violent movement, and, on the 

 other hand, diminished reflex excitability, so that it becomes difficult or im- 

 possible to call forth reflex action. 



504. When we come to study the reflex actions of man we should at 

 first perhaps be inclined to infer that, since in him the spinal cord is so 

 largely used as the instrument of the brain, the independent reflex actions of 

 the cord, at least such as affect skeletal muscles, are in him of much less im- 

 portance than they appear to be in animals, and experience seems to sup- 

 port this view. But it must be remembered that in his case, as we have 

 already stated ( 496), we lack the guidance of experimental results ; we 

 are obliged to trust to the entangled phenomena of disease or to a study of 

 the behavior of the cord while it is still a part of an intact nervous system ; 

 and each of these methods presents difficulties of its own. The movements, 

 which in the intact human body we can recognize as indubitable reflex 

 actions, are, as a rule, simple and unimportant. They are, in by far the 

 greater number of instances, occasioned by stimulation of the skin or of the 

 mucous membrane, for the most part involve a few muscles only, and 

 rarely indicate any very complex coordination. The flexion, followed by 

 extension, of the leg, which is called forth by tickling the sole of the foot, 

 or the winking of the eye when the cornea or conjunctiva is touched, may 

 perhaps be regarded as the type of these movements. A very common 

 form of reflex action is that in which a muscle or group of muscles is 

 thrown into contraction by stimulation of the overlying or neighboring 

 skin, as when the abdominal muscles contract upon stroking the skin of 

 the abdomen or the testicle is retracted upon stroking the inside of the 



