SENSE OF Toueu.
Special Organs of Touch. — The peculiar endowments of the tegumentary surface, which enable us to draw from the impres sions received through it, information of so much more varied and definite a character than we can derive through any of the struc tures which it invests, appear to consist prin cipally, so far as the organ itself is concerned, in its greater sensibility (that is, in its greater aptitude for being affected by slight impres sions), and in its greater power of commu nicating distinct impressions from points in close proximity ; but a large part of our in formation is dependent upon our power of giving motion to the tactile organ, and thus of increasing the force and variety of the impres sions which we derive through its surface, as well as of receiving impressions of an entirely different kind, from the action of the muscles by which that motion is given. Thus, if we simply bring a solid body into contact with the point of the finger, we gain but little information of the nature of its surface, whether rough, smooth, or polished ; and we can judge nothing of its form, except in regard to that part of it in actual contact with the fingers ; and even this is but vaguely ap preciated. This information may be rendered somewhat more precise by pressing the object against the finger ; as we shall then feel the impression made by elevations, points, or rouglinesses, if they be sufficiently prominent and wide apart from each other ; whilst from the degree of muscular force we exert, and from the amount of yielding of which we are conscious in the object itself, we judge of its hardness, softness, elasticity, &c. But our power of discrimination is immensely in creased, when we move the tactile surface on the body to be examined, or vice versa; for, from the succession of impressions then made, we obtain our best idea of the character of the surface of the object ; whilst by the combination of the tactile impressions with the muscular sense, we judge of the relative positions and connections of its different parts, and of the form of the whole. But besides this, we find that impressions may be derived through the skin, which are not re ferable to a mere exaltation of its common sensibility, being apparently of a different cha racter from any of which we become con scious through other structures; such, espe cially, are sensations of temperature. Still there would seem no sufficient cause for rank ing even these in a distinct category from the ordinary tactile impressions ; for the feeling of heat or cold does not differ more from that of roughness or smoothness, than does the colour of the object, as seen by the eye, from its form as distinguished by the same organ.
The different parts of the cutaneous sur face are endowed with tactile sensibility in very different degrees ; and this variation seems closely to correspond with the degree of development of that papillary structure, which may be regarded as the special organ of touch, strictly so called. These papilla' are most elevated and numerous on the tip of the tongue and the points of the fingers ; are less so on the palms of the hands and the soles of the feet; are comparatively small and few on the integument of the limbs, and on several parts of the trunk can scarcely be dis covered at all. They are described by Messrs.
Todd and Bowman as having an average length in man of .0eh of an inch ; and a diameter at their base, where they spring from the cuticle, of about of an inch. Their form is somewhat conical, tapering off to a slightly rounded point. Their surface (after the removal of the epidermis) appears to be composed of the basement membrane of the cutis itself ; and their interior is composed of fibrous tissue, vessels, and nerves. In each papilla we find a small arterial twig, derived from the arterial plexus of the cutis ; this, advancing towards the apex of the papilla, subdivides into two or more capillary yes 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 papillm," as Messrs. Todd and Bowman correctly remark, (loc. 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 papilla', 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 ; 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 papillm themselves, we can affirm that we have distinctly traced solitary tubules ascending among the other tissues of the papillm 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 papillm, 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 papillm."* 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 in 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.