Licence: Public Domain Mark
Credit: Physiology of the nervous system / by Robert B. Todd. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![menial 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 the Junctions of the anterior and posterior roots, a striking difference in t!ie structure of the anterior and posterior horns of the spinal grey matter in wliich 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. Tliese 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 fauciura. 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 broughtto bear on the pupil, by di- recting it to another muscle at the same time, namely, the internal rectus muscle of the eyeball. 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- ment of many actions. This liypothesis 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 ])ressure 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 fi-om 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 orbieular 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-motory act might be prevented by substituting a voluntary act for it; but, in the cases in question, the mind prevents action](https://iiif.wellcomecollection.org/image/b22332807_0050.jp2/full/800%2C/0/default.jpg)
