465 



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FIG. I. Naked axons and terminals in 

 the cornea of the monkey, stained with 

 methylene blue. .1. Beaded axons ram- 

 ifying in the basal part of the epithelium 

 (X 240). B. Epithelium only which has 

 been stripped off substantia propria. 

 Beaded nerve fibers are terminating ex- 

 tracellularly in end beads passing be- 

 tween cells in the middle third of the 

 epithelium (X 400). [From Zander & 

 Weddell (313).] 



At a number of areas in the human body pain has 

 been said to be the only sensation ehcitable in the 

 normal state. If this be true, it has seemed especially 

 reasonable to excite such areas in animals and study 

 the concomitant nervous behavior on the assumption 

 that it may be correlated with pain perception. 

 Appropriate stimulus of the most intensively studied 

 such area, the cornea, has however been shown clearly 

 to evoke other sensations than pain and will be dis- 

 cussed more fully. Even so, pain is the dominant and 

 by far the most readily provoked sensation upon 

 corneal stimulus in man and, in general, animal ex- 

 periments in which the stimulus used would surely 

 bring on pain in a normal man have been useful, 

 especially in analysis of action potentials in nerves. 

 Indeed Beecher(i6) considers that a more dependable 

 relationship has been established between the action 

 of powerful narcotics and the 'experimental pain 

 threshold' in animals than in man. 



END ORGANS FOR PAIN 



Normal Skin 



The finding by Goldscheider (loi) of points on the 

 skin particularly .sensitive to painful stimuli and of 

 other spots which could be stuck painlessly with a 

 fine needle has been followed by efforts to locate a 



particular end organ, the nervous receptor for pain. 

 vonFrey's (273, 274) exhaustive studies with calibrated 

 hairs and thorns led him to insist on a distinction be- 

 tween the spots in which the sole threshold response to 

 a point stimulus was a sense of pain and those in which 

 it was a sense of pressure. Nerves end in the skin in a 

 wide variety of complex patterns or specialized end or- 

 gans (see other chapters for discussion), but the over- 

 whelming majority of the fibers in the skin terminate 

 both in epidermis and dermis without specialized 

 groupings of cells about them, merely as fine naked 

 freely ending axoplasmic filaments in an extracellular 

 position. They interweave i:)ut do not fuse with one 

 another (288). In the corneal epithelium such termi- 

 nals are "disposed in depth throughout its whole 

 extent," as well as "throughout the whole extent of 

 the .substantia propria." [See Zander & Weddell's 

 (313) thorough original studies and analysis of the 

 massive literature on the subject.] Figure i illustrates 

 the appearances in two types of preparation. Weddell 

 and others (personal communication) have also noted 

 a gross variability from week to week in the number of 

 clusters of corneal naked nerve endings simulating in 

 appearance a Krause's end bulb, an observation 

 which indicates that the normal nerve endings may be 

 in a constantly changing dynamic state. 



The multitudinous plexiform endings have been 

 correlated with the multiplicity of ' pain points' found 



