TOUCH CORPUSCLES. 



923 



When the skin of the frog is stimulated by light blows with a tiny 

 blunt-pointed lever, some spots are found, as judged by electrical response 

 of the sensory nerves, to be easily excitable, and others to be inexcitable. 

 Punctiform distribution of the tactual surface seems thus proved in the 

 frog's as well as in the human skin. 1 



As to the tactual sensitivity of the cornea, there exists difference of opinion. 

 Some observers 2 maintain that pure tactual sensations can be elicited from it 

 and from the conjunctiva as well, although all regard the palpebral conjunctiva 

 under the upper eyelid as very poorly tactual. The difference is less a differ- 

 ence between actual observation than of interpretation of observation. The 

 latter turns on the question of the quality of weak sensations evoked by 

 contact stimuli. Some consider even the weakest of these to have unpleasant 

 quality, when elicited from cornea and conjunctiva. If the differentiation 

 between (a) pure tactual sensations and (/?) sensations of contact of ineradic- 

 ably painful quality, be admitted at all, then there can be little hesitation in 

 allowing that the cornea (and parts of the conjunctiva), although richly 

 endowed with " pain spots," are destitute, or almost destitute, of " touch spots." 3 

 The passage of the upper eyelid over the eyeball in blinking is seen but is not 

 felt as touch, except at the moment of meeting of the borders of the two lids. 

 The eyelids being closed, the point of a blunt pencil pressed upon the cornea 

 through the upper eyelid is not felt at the corneal surface, although clear 

 enough on the skin of the eyelid, and although, as the pencil is pressed by 

 one's self, the fingers by secondary touch easily feel the resistance of the hard 

 cornea against the point of the pencil. 



As to the identification of the particular end-organ, among those structurally 

 recognised in the skin, to which " touch " may be specifically assigned, no 

 doubt the short hairs are most sensitive tactual apparatus. To them are 

 distributed nerve fibrils which end in the hair papilla, and others which 

 encircle the hair follicle, in many instances composing a rich ring-like arrange- 

 ment near the level of the openings of the sebaceous glands into the hair follicle. 

 Further, 4 Meissner's corpuscles have for long been regarded as tactual, and 

 with much probability of justice. They seem to take the place of sensory 

 hairs in hairless parts. Hairs are by some 5 regarded as homologues of certain 

 cutaneous sensory apparatus found in fish and amphibians. Such a relation- 

 ship conforms well with a certain degree of physiological analogy that can be 

 discovered between hairs and Meissner corpuscles. 



The following data are useful for comparison with those above given 

 regarding the distribution of touch spots : — 



Average Number of Meissner's Corpuscles to 1 sq. mm. of Skin Surface. 6 



of end phalanx of index finger . . . . .21 



Volar ,, second ,, ,, ..... 8 



surface "I ,, proximal ,, ,, ..... 4 



^ ,, metacarpus of little finger ..... 2 



,, plantar face of end-phalanx of hallux ... 7 



,, middle of sole ....... 2 



One corpuscle occurs on the average in every 35 sq. mm. of skin over the front 

 of the forearm ; von Frey and Kiesow located 99 touch-spots in 6 sq. cm. of the 



1 Steinach, Arch. f. d. ges. Physiol., Bonn, Bd. lxiii. S. 495, etc. 



2 G. Fuchs, Med. Jahrb., Wien, Bd. iv. ; A. Molten, "Inaug. Diss.," Erlangen, 1878 ; 

 H. H. Donaldson, Mind, London, 1885, vol. x. p. 399 ; Max Dessoir, Arch. f. Physiol., 

 Leipzig, 1892, S. 175 ; Nagel, Arch. f. d. ges. Physiol., Bonn, 1895, Bd. lix. S. 570. 



:! Probably there is a considerable amount of individual variation, see Nagel, loc. cit. 



4 "Beitr. z. Anat. u. Physiol, d. Haut," Leipzig, 1853. 



5 Maurer, Morphol. Jahrb., Leipzig, Bde. xviii., xx. 



6 Meissner, "Beitr. z. Anat. u. Physiol, d. Haut," Leipzig, 1853. 



