It is plain, then, that the spinal cord,although apart from the encephalon it takes no share in sensations and voluntary actions, (for then, indeed, these phenomena cannot take place as far as regards the trunk and extremities,) while united with the encephalon participates fully in sensori-volitional actions, and its integrity is quite necessary to the perfection of those actions.
I repeat that we are not justified in supposing that the tnind localises itself exclusively in some or all of the gangliform bodies, the assemblage of which constitutes the encephalon ; but this we may assert, with perfect justice, that when the cord has been separated from the encephalon, the mind appears as it were to cling to the latter organ, and to lose all its connection with the former.
Does then the cord, under these circum stances, lose all its power ? Does it, when sepa rate from the encephalon, shew no indication of acting as a nervous centre ? Undoubtedly it does show abundant indications. A series of actions, which had attracted the notice of several physiologists, are still capable of being developed through the instrumentality of the whole cord or of any portion of it, the nerves of which may remain uninjured both as to their central and periphera4 connections.
Phenomena of this nature may be produced in all vertebrate animals. They are, however, especially marked in the cold-blooded classes, in consequence of the more enduring character of the nervous force in those creatures than in the warm-blooded. Hence frogs, salamanders, snakes, turtles, fishes, have been generally selected by physiologists for exhibiting these phenomena. In the young of warm-blooded animals they are more manifest than in adults of the same class.
If a frog be pithed by dividing the line of junction of the medulla oblongata with the spinal cord, the following effects may be ob served. After the first disturbance, geneml convulsions, &c., consequent upon the division of the cord, the animal, if placed on a table, will assume his ordinary position of rest. In some cases, however, frequent combined move ments, much resembling acts of volition, will take place for a longer or shorter time after the operation. When all such disturbance bas ceased the animal remains perfectly still and as if in repose, nor does it exhibit the slightest appearance or give the least expres sion of pain or suffering. It is quite unable to produce any spontaneous or voluntary move ment of parts supplied with nerves from be low the section, that is, of the trunk or extre mities. However one may try to frighten it,
it remains in the satne place and posture. The only appearance of voluntary motion is the winking of the eyelids, which, however, proba bly is not excited by the will. lf, now, a toe be pinched, instantly the limb is dmwn up, or the animal seems to push away the irritating agent, and then draws up the leg a,min into its old position. Sometimes a stimulus of this kind excites both leg,s, and causes them to be thrown violently backwards. A similar movement al most constantly follows stimulation of the anus. If the skin be pinched at any part, some neighbouring muscle or muscles will be thrown into action. Irritation of the anterior extre mities will occasion movements of them ; but it is worthy of note that these movements are seldom so energetic as those of the posterior extremities.
We may remark here, that phenomena of this kind are not confined to the trunk and extremities, which are supplied only by spinal nerves. The head and face, with which the encephalon remains in connection, exhibit similar actions. The slightest touch to the marg,in of either eyelid or to the surface of the conjunctiva causes instantaneous winking; the attempt to depress the lower jaw for the purpose of opening the month is resisted ; and the act of deglutition is provoked by applying a mechanical stimulus to the back of the throat..
Actions similar to those which take place in the decapitated frog, occur in the human subject when the spinal cord has been separated from its encephalic connections by disease or acci dent. In such cases it is found that although the will cannot move the paralysed parts, the lower extremities for instance, movements do occur iu them of which the individual is wholly un conscious, and which lie is utterly unable to pre vent. Sometimes these take place seemingly quite spontaneously; at other times they are excited by tlie application of a stimulus to some surface supplied by spinal nerves. The move ments of this kind, which seem to occur spontaneously, exhibit so close a resemblance to voluntary actions as to render it impossible to distinguish them, did not the consciousness of the patient in some cases assure him of the in active state of his will in reference to them.