of the skin; usually being larger where the papillæ in which they lie are well developed. The axis cylinders are said to end in swellings called tactile cells. 2. End bulbs (Krause) are smaller than the last and are less generally distributed over the surface of the body, being localized to certain parts. They are chiefly found in the conjunctiva and mucous membranes of the mouth and external generative organs. They consist of a little vesicle containing some fluid; a few large nucleated cells. The axis cylinder terminates between the cells, FIG. 212. Drawing of termination of nerves on the surface of the rabbit's cornea. a, Nerve fibre of sub-epithelial network; b, Fine fibres entering epithelium; c, Intra-epithelial network. (Klein.) the membrane which forms the vesicle of the bulb being fused with the sheath of the nerve. Many different shapes and varieties of these bodies have been described, but there seems to be no definite morphological or physiological distinction between the varieties. 3. Touch cells (Merkel), found in the deeper layers of the epidermis of man as well as in the tongues of birds, are large cells of the epithelial type with distinct nuclei and nucleoli. Frequently they are grouped together in masses and surrounded by a sheath of connective tissue; in which condition they resemble touch corpuscles. 4. Free nerve endings occur on the surface of the epithelium of the mucous membranes, and are seen on the surface of the cornea (Cohnheim). Here delicate, single strands of nerve fibrils can be seen after gold staining, passing between the epithelial cells and ending at the surface in very minute blunted points or knobs. Naked nerve fibrils have also been traced into the deeper layers of the epidermis of the skin, where they end among the soft cells of the mucous layer, either in branched cell-like bodies (Langerhans) or delicate loops (Ranvier). In the subcutaneous fat tissue and in parts remote from the surface some sensory nerves terminate in large bodies, easily visible to the naked eye, called— 5. Pacinian Corpuscles.-They are ovoid bodies made up of a great number of concentrically-arranged layers of material, of varying consistence, with a collection of fluid in the centre, in which an axis cylinder ends. There is no doubt that they are the terminals of afferent nerves, but if they are connected with the sense of touch, which is doubtful from their distribution, it is unknown to what special form of sensation they are devoted. From their comparatively remote relation to the skin, lying some distance beneath it and not in it, like the other endings mentioned, they are probably connected with the appreciation of pressure sensations rather than those more properly called tactile. The sense of touch must be carefully distinguished from ordinary sensibility or the capability of feeling pain, which is not a special but a general sensation, and is received and transmitted by different nerve channels. This we know from the facts, that the mucous passages in general can receive and transmit painful but not tactile impressions, and that in the spinal cord the sensory and tactile impulses are probably conveyed by distinct tracts. Certain narcotic poisons destroy ordinary sensation without removing the sense of touch. This effect is also brought about by cold, when the fingers are benumbed; gentle contact excites tactile impressions, while the ordinary sensations of pain can only be aroused by severe pressure. However, most of the nerves we call sensory nerves convey tactile impressions, and, speaking generally, those parts of the outer skin which have the keenest tactile sense are also those most ready to excite feelings of pain. The intensity of the stimulation for the sense of touch must be kept within certain limits in order to be adequate, i. e., capable of exciting the specific mental perceptions. If the stimulus exceed these limits, only a general impression, approaching that of pain, is produced. The power of forming judgments by touch differs very much in different parts of the body, being generally most keen where the surface is richest in touch corpuscles, namely, the palmar aspect of the hands and feet, and especially the finger tips, tongue, lips, and face. When we feel a thing in order to learn its properties, we make use of all the qualities of which our sense of touch is made up. We estimate the number of points at which it impinges on our finger tips, rub it to judge of smoothness, press it to find out its hardness, and at the same time gain some knowledge of its temperature and power of absorbing heat. To get a clear idea of our complex sense of touch, we must consider each kind of impression separately. SENSE OF LOCALITY. By this is meant our power of judging the exact position of any point or points of contact which may be applied to the skin. Thus, if the point of a pin be gently laid on a sensitive part of the skin we know at once when we are touched, and if a second pin be applied in the same neighborhood, we feel the two points of contact and can judge of their relative position. When we feel anything, we receive impulses from many points of contact bearing varied relationships to each other, and thus become conscious of a rough or smooth surface. The delicacy of the sense of locality differs very much in different parts of the skin. It is most accurate in those parts which have been used as touch organs during the slow evolution of the animal kingdom. The method of testing the delicacy of the sense of locality is that of applying the two points of a compass to different parts of the skin, and by varying their position, experimentally determine the nearest distance at which the two points give rise to distinct sensations. The following precautions must be attended to in carrying out this experiment: 1. The points must be simultaneously applied, or two distinct sensations will be produced at abnormally small distances. 2. The force with which the points are applied must be equal and minimal, because excessive pressure causes a diffusion of the stimulus and a blurring of the tactile sense. 3. Commencing with greater and gradually reducing the distance of the points enables a person to appreciate a less separation than if the smaller distances were used at first. 4. The duration of the stimulus; two points of contact being distinguished at a much nearer distance if the points be allowed to rest on the part, than when they are only applied for a moment. 5. The temperature and material of the points should be the same. 6. Moisture of the surface makes it more sensitive. 7. Previous or neighboring stimulation takes from the accuracy of the sensations produced. 8. The temperature of the different parts of the skin should be equal, as cold impairs its sensibility. The following table gives approximately the nearest distances at which some parts, which may be taken as examples of the more or less sensitive regions of the skin, can recognize the points of contact by their giving rise to two distinct sensations:— If one point of the compass be applied to the same spot, and the other moved round so as to mark out in different directions the limits at which the points can be distinguished as separate, we get an area of a somewhat circular form, for which the name sensory circle has been proposed. It would be convenient to explain this on the simple anatomical basis that the impressions of this area were carried by one nerve fibre to the brain, and thus but one sensation could be produced in the sensorium. We know this cannot be the true explanation, from the following facts: 1. No such anatomical relationship is known to exist. 2. By practice we can reduce the area of our sensory circles in a manner that could not be explained by the development of new nerve fibres. 3. If the two points of the compass be placed near the edges of two well-determined neighboring sensory circles, and so in relation with the terminals of two nerve fibres, they will not give distinct impressions; they require to be separated as much as if they were applied within the boundary of one of the circles where they also give rise to the double perception. To explain better the sense of locality, it has been supposed that sensory circles are made up of numerous small areas, forming a fine mosaic of touch fields, each of which is supplied by one nerve fibre, and that a certain number of these little fields must intervene between the stimulating points of the compass in order that the sensorium be able to recognize the two impulses as distinct. For, although every touch field is supplied by a separate nerve fibril which carries its impulses to the brain, and is therefore quite sensitive, the arrangement of the cells in the sensorium is such that the stimuli carried from two adjoining touch fields are confused into one sensation. Thus, when an edge is placed on our skin, we do not feel a series of points corresponding to the individual fields with which it comes in contact, but the confusion of the stimuli gives rise to an uninterrupted sensation, and we have a right perception of the object touched. THE SENSE OF PRESSURE. There seems to be a reason for separating the perception of differences in the degree of pressure exercised by a body from the simple tactile or local impression. If we support a part of the body so that no muscular effort be called into play in the support of an increasing series of weights placed upon the same area of skin, we can distinguish tolerably accurately between the different weights. It has been found that if a weight of about |