THE NERVOUS SYSTEM 317 



system, ring out clearly, and hence the mind locates approxi- 

 mately the " pain " of the whitlow, the muscle-ache, the 

 decayed tooth. Sufferers from toothache are familiar with 

 the phenomenon of the spread of pain from a definite spot 

 to the whole jaw or the whole side of the head, dependent 

 upon the spread of the pain-agitation from the segment of the 

 axis of the brain in which the dental nerve ends to neighbour- 

 ing segments. Our ability or inability to localize a pain 

 does not depend upon the presence or absence of pain- 

 nerves, but upon the existence or non-existence of nerves 

 coming from the same organ, or from its neighbourhood, and 

 capable of conveying impulses to the seat of consciousness. 

 In passing through the part of the spinal cord or of the axis 

 of the brain which is disturbed by the influence exercised by 

 a damaged organ, silent impulses acquire force sufficient to 

 render them audible, and combine with the pain to produce 

 a feeling which consciousness can analyse, to a certain extent. 

 Informed as to its whereabouts by these accentuated sensations, 

 consciousness recognizes a sense of pain limited in its topo- 

 graphical extension. 



Sneezing when a bright light falls upon the eye is a curious 

 illustration of the exaggeration of the effectiveness of sensory 

 impulses when they happen to be poured into an agitated 

 segment of grey matter. About one person in every three is 

 affected in this way. A friend of the writer, who was par- 

 ticularly sensitive, rising in the night because he heard his 

 child cry, three times lighted a candle and three times sneezed 

 it out before he could watch the application of match to wick 

 without suffering from a nerve-storm. Some nervous dogs 

 especially fox-terriers are very liable to this neurosis. Many 

 persons who do not sneeze feel, when the sunshine stimulates 

 their retinae, a tickling in the nose. Again the illusion is to be 

 traced to the door of the fifth nerve the sensory nerve of the 

 whole of the face. The nose is the true tip of the body. 

 Morphologically it is anterior to the eyes. Just as the fifth 

 nerve extends its distribution to the nose, so also its root- 

 fibres extend their connection within the axis of the brain 

 forwards, until they traverse the mid-brain, the primary centre 

 of the optic nerve. A bright light, by stimulating the optic 

 nerve, sets up a commotion in the mid-brain. The ordinary 



