1166 



TOUCH. 



sels; and these, forming loops whose con- 

 vexity lies in the summit of the papilla, re- 

 unite into venous radicles, which discharge 

 their blood into the venous plexus of the 

 cutis. " The vascularity of the papillae," as 

 Messrs. Todd and Bowman correctly remark, 

 (Joe. cit.}, " is such, that their presence and 

 relative size may be determined simply by the 

 depth of the colour imparted to the skin by 

 a good injection of its vessels ; the vascularity 

 of the integument is, therefore, in general 

 terms, proportioned to its perfection as an 

 organ of touch." With regard to the nervous 

 supply of these papillae, however, it is less 

 easy to speak with confidence. It is derived, 

 like the sanguiferous, from the plexus in the 

 substance of the cutis, lying parallel to the 

 surface ; the tubular fibres ascend, to all ap- 

 pearance singly, from this plexus into the 

 papillae ; but their mode of termination or 

 return are not distinguishable with certainty. 

 The following are the results of the in- 

 quiries of Messrs. Todd and Bowman on this 

 point. " In regard to the presence of nerves 

 in the papillae themselves, we can affirm that 

 we have distinctly traced solitary tubules 

 ascending among the other tissues of the 

 papillae about half-way to their summits, but 

 then becoming lost to sight, either by simply 

 ending, or else by losing the white substance 

 of Schwann, which alone enables us to dis- 

 tinguish them in such situations from other 

 textures We have in numerous in- 

 stances failed to detect any nerves at all within 

 the papillae, when such were plainly visible at 

 their base, and when, consequently, the che- 

 mical agent employed could scarcely have 

 destroyed their characteristic structure had 

 they been present. We incline to the belief 

 that the tubules, either entirely or in great 

 measure, lose the white substance when 

 within the papillae."* With these statements, 

 so far as they go, the writer's own observa- 

 tions are in entire accordance ; but he thinks 

 that from the appearances presented by sec- 

 tions examined by reflected instead of by 

 transmitted light, it may be inferred that the 

 nervous tubules fn the tactile papillae undergo 

 a change somewhat similar to that which is 

 said by Wagner to take place in the nervous 

 tubuli of muscle ; namely, that whilst the 

 white substance of Schwann is not traceable 

 beyond a certain point, the central axis is 

 continued further, and that this breaks up 

 into minuter fibrillae, which form loops, like 

 those of the capillary blood vessels, returning 

 into the tubular fibre itself. 



In the lower animals, as in man, it may be 

 observed that the papillary structure is espe- 

 cially developed on those parts of the tegu- 

 mentary surface which are especially endowed 

 with tactile sensibility, and the impressions 

 received through which are of important use 

 in finding the movements of the instruments 

 of locomotion. Thus in the quadrumana 

 generally, both the hands and feet are thickly 

 set with papillae ; and in those which have a 



* Op. cit. p. 412. 



prehensile tail, the surface of this organ pos- 

 sesses them in abundance. In the carnivorous 

 and herbivorous mammalia, whose extremi- 

 ties are furnished with claws or encased in 

 hoofs, we find the lips and the parts sur- 

 rounding the nostrils to be the chief seat 

 of tactile sensibility, and to be copiously 

 furnished with papillae ; this is especially the 

 case with those which have the lips or nos- 

 trils prolonged into a snout or proboscis, as 

 in the pig, the rhinoceros, the tapir, and the 

 elephant. In the mole, too, the papillary 

 structure is remarkably developed at the ex- 

 tremity of the snout. The only part of the 

 skin of birds on which tactile papillae have 

 been discovered is on the under surface of 

 the toes, and on the web of the palmipedes, 

 where they obviously receive impressions, 

 which guide the prehensile and other move- 

 ments of the feet. It is probable, however, 

 that the very sensitive skin which covers the 

 greater part of the mandibles in the duck 

 tribe, is furnished with papillae ; the tactile 

 impressions received through this part, when 

 the bill is plunged into mud &c., being the 

 chief means by which the presence of food is 

 discovered. In many lizards a papillary struc- 

 ture is found on the under surface of the 

 toes ; and in the chameleon it exists, also, on 

 the integuments of its prehensile tail. In the 

 soft skinned batrachia, an imperfect papillary 

 structure is more extensively diffused over 

 the surface ; but on the thumb of the male 

 frog, and probably on that of other batrachia, 

 there is an extraordinary development of pa- 

 pillary tissue at the season of sexual excite- 

 ment, large papillae being formed all over it. 

 This organisation is obviously connected with 

 the extraordinary prehensile propensity which 

 is then displayed by the animal, and which 

 enables him to keep the female in his grasp 

 during the whole period of the discharge of 

 the ova, in a manner which no voluntary effort 

 could effect. In serpents and chelonians, no 

 papillary apparatus has yet been detected ; 

 and in fishes and invertebrata its presence has 

 not been ascertained, although it would ap- 

 pear that certain parts, especially the tenta- 

 cula around the oral orifice, are endowed with 

 a high degree of tactile sensibility.* 



But it is not only on the tegumentary sur- 

 face of the exterior of the body, that tactile 

 sensibility is particularly acute ; nor is the pa- 

 pillary apparatus restricted to it alone. In 

 the tongue of man, we find the sense of touch 

 remarkably developed, especially at its tip ; 

 and of the papillae with which its surface is 

 beset, it is probable that some are the instru- 

 ments of tactile sensibility ; whilst others 

 minister to the gustative sense. (See TASTE.) 

 So it is probable that in all animals which 

 have a soft fleshy tongue, furnished with pa- 

 pillae, and serving as the organ of taste, this 

 organ is the instrument of tactile sensibility also. 



* The movements of these organs, or such as are 

 excited through contact with them, can scarcely be 

 in themselves regarded as a sufficient indication of 

 their tactile sensibility, as they may be purely reflex, 

 without involving consciousness of the impression. 



