722F 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



mental stimuli, and the mind loses all control ; 

 this is the case in poisoning by strychnine, in 

 tetanus, in convulsions. 



The difference of structure of the anterior 

 and posterior horns of the vesicular matter of 

 the spinal cord may be appropriately referred 

 to as indicating a difference in functions be- 

 tween these horns. The anterior horns contain 

 large caudate vesicles of a remarkable and 

 peculiar kind, containing a considerable quan- 

 tity of pigmentary matter ; the posterior horns 

 resemble very much in structure the vesicular 

 matter of the cerebral convolutions and of 

 other parts of the cerebrum, and do not contain 

 caudate vesicles, except near the base. Here, 

 then, we find associated with the well-attested 

 difference in tine junctions of the anterior and 

 posterior roots, a striking difference in the 

 structure of the anterior and posterior horns of 

 the spinal grey matter in which they are re- 

 spectively implanted. 



We gain from this hypothesis that which 

 neither of the others could supply, namely, an 

 explanation of the influence of emotion on 

 limbs paralysed to volition. Mental emotion 

 excites a change in the brain, probably in that 

 part which forms the upper and posterior por- 

 tion of the mesocephale : this change is readily 

 propagated to the spinal grey matter through 

 the olivary columns, independently of the py- 

 ramidal fibres. The spinal grey matter being 

 excited, the nerves implanted in it are stimu- 

 lated, and motions are produced closely resem- 

 bling those which the will can develope. 



We have noticed that the will can control 

 reflex or other physical nervous actions. When 

 the influence of the will is suspended, reflex 

 actions may be more easily excited. These 

 facts admit of the most obvious explanation by 

 the hypothesis under examination. 



Some reflex actions are imperfectly control- 

 lable by the will ; such as the contraction of 

 the pupil, and the movement of deglutition at 

 the isthmus faucium. This, however, cannot 

 be cited as at all opposed to the view we are 

 advocating ; for there is nothing in this hypo- 

 thesis repugnant to the idea that certain nerves 

 may be connected in the nervous centres with 

 masses of vesicular matter over which the will 

 usually exercises little or no control, and which, 

 perhaps, may have but a slight connection 

 with the centre of volition through commissural 

 fibres. Still, respecting the two actions above- 

 mentioned, it must be remarked that in de- 

 glutition the mental influence is not sufficient 

 by itself: we cannot perfectly contract the fau- 

 ces, if food or some other physical stimulus be 

 not present; the double stimulus physical, as 

 of the food, and mental, the will appears 

 necessary for the perfect performance of this act. 

 In the action of the pupil, the mental stimulus 

 can only be brought to bear on the pupil, by di- 

 recting it to another muscle at the same time, 

 namely, the internal rectus muscle of theeyeball. 

 When the eyeball is directed toward the nose, 

 the pupil is usually simultaneously contracted. 



A double stimulus, mental and physical, 

 appears to be necessary to the perfect develope- 

 nient of many actions. This hypothesis offers 



a ready explanation of the way in which the 

 two stimuli may combine to promote the same 

 action. The mental stimulus acts directly on 

 the vesicular matter, the physical is propagated 

 to it by sensitive nerves ; and thus both acting 

 on the same region of vesicular matter excite 

 the same motor nerves. We have already no- 

 ticed how this takes place in deglutition at the 

 isthmus faucium. In locomotion there can be 

 no doubt that the double stimulus is in opera- 

 tion : the degree of contraction of the muscles 

 of the lower extremities necessary to maintain 

 the superincumbent weight is obtained by the 

 physical stimulus of pressure against the soles 

 of the feet, where the skin is peculiarly fitted 

 for the reception of such a stimulus; but the 

 movements of the limbs, and the harmonizing 

 association of the muscular actions, are effected 

 by mental influence. The pressure against the 

 soles is felt, however, and the skin of the soles 

 is known to be highly sensitive ; and the same 

 nerve-fibres which excite the sensation stimulate 

 the vesicular matter in which the motor nerves are 

 implanted. In many actions of familiar oc- 

 currence the voluntary effort is greatly enhanced 

 by the simultaneous application of a physical 

 stimulus to a part of the surface which is sup- 

 plied 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, al- 

 though of no greater bulk, is yet so irregular 

 in surface as not to allow of such intimate con- 

 tact 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 sen- 

 sation, and excites motion which is at once the 

 result of the physical impression, 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, aris- 

 ing from the contact of air with the conjunctiva, 

 and the diminution of temperature from eva- 

 poration on the surface of that membrane, 

 which at first caused but a slight sensation, 

 produces pain ; the physical stimulus over- 

 comes the mental resistance, and causes con- 

 traction of the orbicular muscle. And it may 

 be remarked further, 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 explanation 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-raotory act might be prevented 

 by substituting a voluntary act for it ; but, in 

 the cases in question, the mind prevents action 



