In the action just referred to, as well as in all other instances of reflex actions which the will can prevent, no satisfactory axplanation 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 prevcnusl. by substitutmg a voluntary act for it ; but,. in the cases in question, the tnind prevents act= altogether, notwithstanding the exciting influ ence 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 hemi plegia from diseased brain, they are generally very obvious in the lower extremity, but to tally absent in the upper. This, the advocates of the excito-motory 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 from which the nerves of the upper extremities arise, as nearer the seat of lesion, than upon the lumbar segment. But another explanation of this important fact may be offered, which is equally satisfactory, and more accordant with other phenomena. A certain disposition of the nerves npon the tegu mentary surface is as necessary for the develope ment if reflex actions as of sensations ; and these movements will be more or less easily manifested, according as this organization of the nerves on the surface is more or less perfect.
That disposition of the cutaneous nerves which renders the surface easily excitable by titillation seems most favourable to the deve lopement of these actions. Hence, there is no place where they are more readily excited than in the lower extremities by stimulating the soles of the feet or the intervals between the toes, .both of which situations are highly susceptible of titillation. At the isthmus faucium the slightest touch on the surface excites a move ment of deglutition; and this touch, at the saline time, produces a very peculiar sensation of tickling, quite distinct from that which may be excited at other parts of the pharynx, or mouth. When this part of the mucous mem
brane is in a state of irritation as an effect of coryza, this tickling sensation is present, and repeated acts of swallowing are provoked.
Two facts may be stated here, which illus trate the position above laid down respecting the necessity of a certain disposition of the nerves on the tegumental surface, for the de velopement of retlex actions. The first is one which has been noticed by Volkmann, and which I have repeatedly observed, namely, that in frogs, and other animals, reflex actions are readily excited by stimulating the feet ; but irritating the posterior roots of the spinal nerves, which supply those parts, is not sufficient for this purpose. I have already remarked that in numerous experiments upon the posterior roots of the nerves movements have not been ex cited whilst they have been subjected to irrita tion, except when galvanism was employed, which, being diffused, affected the cord itself : the recorded statements of most modem ex perimenters agree in the main with this state ment. The Second fact is this: in the male frog the developement of a papillary structure on the skin of the thumb seems to have refe rence to the excitation of the physical power of the cord, to enable the animal to grasp the female without the necessity of a. prolonged exercise of volition. Stimulating the fingers will scarcely produce reflex actions, but the slightest touch to the enlarged thumb will cause the animal to assume the attitude of grasping. If the papillw be shaved off the thumb, its power of exciting these actions is instantly lost.
When the polarity of the cord is greatly ex cited by strychnine or other substances, or when tetanus exists, all parts of the surface are equally capable of exciting reflex actions. The least touch will cause them, not only in the limb touched, but in all that side of the trunk, or even throughout the whole body. So general is the excitation, that the least im pression made on the peripheral extremity of a sensitive nerve in any part of the body is instantly converted into muscular spasm, more or less general. A slight current of air, in tetanus, is sufficient to excite general 'spasm. Miller remarks that, in such states of the cord, the reflex actions excited by stimulating the nerves themselves are much less than those produced by excitation of the surface.