334 INNERVATION. [CHAP. XI. 



physical stimulus to a part of the surface which is supplied with 

 nerves from the same region of the cord. The horseman feels more 

 secure when his legs are in close contact with the horse's flank. We 

 gain a much firmer hold of an object which adapts itself well to 

 the palmar surface of the hand, than of one which, although of no 

 greater bulk, is yet so irregular in surface as not to allow of such 

 intimate contact with the palm. Closure of the eyelids in winking 

 is an action of similar kind, resulting from a physical stimulus, which 

 in the perfect state of the cerebro- spinal centre produces sensation, 

 and excites motion which is at once the result of the physical im- 

 pression, and of the exercise of volition provoked by the sensation. 

 Every one must be conscious that he exercises considerable control 

 over the movements of his eyelids, and that it requires a great effort 

 to prevent winking for a certain period. At length, however, the 

 physical impression, arising from the contact of air with the con- 

 junctiva, and the diminution of temperature from evaporation on the 

 surface of that membrane, which at first caused but a slight sen- 

 sation, produces pain ; the physical stimulus overcomes the mental 

 resistance, and causes contraction of the orbicular muscle. And it 

 may be remarked farther, that the closure of the lids by voluntary 

 effort is much more powerful if a stimulus be applied at the same 

 time to the conjunctival surface, than if left solely to the exercise 

 of the will. 



In the action just referred to, as well as in all other instances of 

 reflex actions which the will can prevent, no satisfactory explana- 

 tion of this controlling power of the mind can be given by Dr. Hall's 

 hypothesis Do the volitional fibres exceed in number the excito- 

 motory ? If this were admitted, then we could understand that an 

 excito-motory act might be prevented by substituting a voluntary 

 act for it ; but, in the cases in question, the mind prevents action 

 altogether, notwithstanding the exciting influence of the impression. 

 The true explanation seems to be, that the mind can exert upon 

 the vesicular matter a power which can prevent the exercise of that 

 change, or neutralise the change, without which the motor fibres 

 will not be affected by a physical stimulus. 



Reflex actions are more manifest in some situations than others : 

 thus, in cases of hemiplegia from diseased brain, they are generally 

 very obvious in the lower extremity, but totally absent in the upper. 

 This, the advocates of the excito-motary theory ascribe to a paucity 

 of excito-motory fibres in the latter limb, and to a larger amount of 

 them in the former. Or, it has been attributed to the greater and 

 more enduring influence of shock upon that segment of the cord 



