CHAP. XI.] MANY ACTIONS NEED A DOUBLE STIMULUS, 333 



will produce the same movements which previously were excited by 

 voluntary impulse. Thus, tickling the soles of the feet, in a person 

 asleep, excites movements which doubtless are of the reflex kind ; 

 but the same stimulation, in a person awake, will give rise to precisely 

 the same movements, which he is conscious are, at least in a great 

 degree, voluntary. 



Some reflex actions are imperfectly controllable by the will ; of 

 which the contraction of the pupil, and the movement of deglutition 

 at the isthmus faucium, are examples. It is remarkable, however, 

 that the will may give rise to these actions by associating others 

 with them : the pupil may be contracted at will, by directing the 

 eye inwards ; and the fauces may be contracted by bringing some 

 saliva in contact with them. In the latter case, the stimulus of 

 volition alone is not sufficient to excite the movement ; the addition 

 of a physical stimulus is likewise necessary : and, in the former, 

 the excitability of the fibres of the third nerve by a mental stimulus 

 may be materially modified by their re-association with vesicular 

 matter in the ophthalmic ganglion. 



There is nothing in this hypothesis repugnant to the idea, that 

 certain nerves may be connected in the centres with masses of 

 vesicular matter over which the will usually exercises little or no 

 control, and which perhaps, have but a slight connexion with the 

 brain through commissural fibres. Facts like those instanced in 

 the preceding paragraphs may be accounted for on such a supposition 

 as this. This supposition may be required to explain some of the 

 actions of nerves connected with the medulla oblongata, the vagus 

 for instance, but certainly not of spinal nerves. 



It is probable that in many actions the double stimulus, mental 

 and physical, is necessary to their perfect development. The 

 former is excited by the mind acting on the vesicular matter ; the 

 latter is propagated at the same time, by sensitive nerves, to the same 

 region of vesicular matter ; and both simultaneously influence the 

 same motor fibres. In locomotion, it seems probable that this is the 

 case : the degree of contraction of the muscles necessary to main- 

 tain the superincumbent weight is obtained by the physical stimulus 

 of pressure against the soles of the feet ; but the movements of the 

 limbs, and the harmonizing association of the muscular actions, 

 is effected by mental influence. The pressure against the soles is 

 felt, however ; and the same nerve-fibres which excite the sensation, 

 stimulate the vesicular matter in which the motor nerves are im- 

 planted. In many actions of familiar occurrence, the voluntary 

 effort is greatly enhanced by the simultaneous application of a 



