SNEEZING, SEA-SICKNESS, PAIN 433 



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 pro- 

 topathic 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 in- 

 teresting. 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 con- 

 tact. 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 con- 

 nected 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 conscious- 

 ness 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. 

 Eoot-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 func- 

 tions, acquire, when the hind-brain is pain-conditioned by impulses 

 from the semicircular canals, a terrifying import, causing the ex- 

 plosion of numberless motor neurones. The stomach sneezes, with the 

 zealous support of muscles of the throat, chest and abdomen. In the 



vol. xxx vi.— 30. 



