Page:Popular Science Monthly Volume 76.djvu/437

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though bound up with the fifth nerve, extend forwards within the cerebrospinal axis to the very front of the mid-brain. A remarkable state of affairs is thus established. The mid-brain receives the protopathic nerves of the eye and the root-fibers of the sensory nerves of the nostrils. When excessive stimulation of the retina by bright light sets up a pain-condition in the mid-brain the every-moment impulses ascending from the nostril acquire undue importance. It is as if they had been increased in intensity by a pinch of snuff. Their urgency causes the reflex by which irritating substances are expelled.

Some persons merely feel a tickling in the nose when they look at a bright light, but do not sneeze. This phenomenon is extremely interesting. It proves that stimuli "adequate" to impress nerve-endings are not necessarily "adequate" to arouse consciousness. External forces incessantly press the button with sufficient energy to make contact. At each pressure a bell rings in the chamber of consciousness, but, if it is to attract attention, it must ring more loudly. The stimuli which gave rise to a tickling feeling were not originated nor intensified by the light which fell upon the retina. The mid-brain through which impulses passed to reach the cortex was rendered more conductile.

In normal conditions no pain results from stimulation of the retina, however severe; because the nerve-fibers which convey visual impulses from this highly specialized sense-organ, are connected, not with the mid-brain, but with the optic thalamus and the occipital cortex. It would stultify so highly specialized a sense, were its news admixed with, or modified by any influence or information not directly connected with its proper function.

Sea-sickness is another illustration of the effect upon reflex action of central agitation due to impulses which do not appear in consciousness however voluminous the sensations may be of which they are the indirect cause. The nerve which is concerned with the adjustment of the position of the body is a constituent of the auditory nerve. It comes from the semicircular canals. Never under any circumstances do the impulses which originate in these organs of orientation enter consciousness; but when a ship begins to roll, or worse, to heave, they churn up the gray matter of the hind-brain until its conductivity is so affected as to demonstrate their urgency beyond misunderstanding. Root-fibers of the vagal nerve traverse the hind-brain much in the same way as root-fibers of the fifth nerve traverse the mid-brain. Habitual, every-moment impulses ascending from the stomach by the vagal nerve, for the routine regulation of its purely domestic functions, acquire, when the hind-brain is pain-conditioned by impulses from the semicircular canals, a terrifying import, causing the explosion of numberless motor neurones. The stomach sneezes, with the zealous support of muscles of the throat, chest and abdomen. In the