912 REFLEX ACTIONS. [BOOK m. 



of consciousness, which we may, in a way, compare 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 involved 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 intelligence, and cannot be appealed to as explaining 

 the ' choice ' spoken of above. 



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

 need hardly say, dependent on the intrinsic condition of the cord. 

 The action of strychnia 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 excitability so that a mere touch 

 may produce a violent movement, and on the other hand diminished 

 reflex excitability so that it becomes difficult or impossible to call 

 forth a reflex action. 



591. 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 importance than they 

 appear to be in animals; and experience seems to support this 

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

 already stated ( 583), we lack the guidance of experimental results; 

 we are obliged to trust to the entangled phenomena of disease or 

 to a study of the behaviour 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 



