Page:Popular Science Monthly Volume 26.djvu/19

From Wikisource
Jump to: navigation, search
This page has been proofread, but needs to be validated.
9
MIND AND THE NERVOUS SYSTEM.

made upon any one of the organs of the senses without a higher mental operation being performed. This is especially the case when the perception is of such a character as to be irritative. Thus, if an exceedingly bright light be allowed to impinge upon the retina, the brows are corrugated, and, if it be still more intense, the eyelids are closed so as to shut it out altogether; if a very loud noise strikes upon the tympanum, the head is turned so as to prevent the undulations of the atmosphere reaching the ear in full force; if the skin be irritated, the part is, if possible, drawn away, and, if the irritation be so great as to excite pain, the whole body is thrown into contortions and efforts are made to escape. Some of these movements appear to be involuntary, and even to be performed in direct opposition to the will, and then they are said to be reflex—that is, that they are the result of the conversion of a sensation into a motor impulse without the accompanying action of any ganglion, the action of which is the evolution of volitional force.

Now, it is very true that some of the actions in question are apparently altogether involuntary, and are thus true reflex movements, and it is no difficult matter to separate them from those other which are clearly volitional, determinate, and performed with a definite purpose in view. If, for instance, an irritative substance be applied to the interior of the nostril, the action which we call sneezing is produced. This consists of a spasmodic contraction of certain muscles by which the air in the lungs is forcibly expelled through the nostrils. It is automatic and preservative in character, the object being to get rid of the offending substance. It is always performed in the same way, the muscles brought into action are always the same, and it is spasmodic, sudden, and without deliberation or judgment, so far as we can determine from our own consciousness. Again, if the soles of the feet are tickled, they are drawn away, although it is possible for the impulse to remove them to be restrained by the exercise of the will, and, indeed, some individuals can prevent sneezing by strong volitional power evolved from the higher ganglia of the brain. But let us suppose the case of a man with a disease of the upper part of the spinal cord of such a character as to prevent its conveying volitional impulses from the brain to the muscles of the lower limbs; now let the soles of the feet be tickled, and we shall find that they are drawn away, and generally with very much more force than when the brain is allowed to act. Such a movement is probably one of true reflex character; it is spasmodic and indeterminate, being more extensive than is necessary. But let us go still further in our suppositions, and imagine that in such a case the mere drawing away of the foot was not sufficient to avoid the irritation, and that the individual deliberately lifted up the other foot in the attempt to remove the offending object, and that this action not proving adequate, he made two or three leaps in order to escape. What would we call these movements? Would they not be