Although the end organs of the nerves of the skin are the simplest of all those belonging to the apparatus of special sense, yet we have a very imperfect knowledge of their immediate relationships to the different qualities of touch impressions. We know of several different nerve endings apparently adapted to the reception of certain impressions, but of the exact kinds of stimuli that affect these different terminals we are ignorant.

End bulb from human conjunctiva, treated with osmic acid, showing cells of core.

Fig. 210. End bulb from human conjunctiva, treated with osmic acid, showing cells of core. {Longworth).

a, Nerve fibre; b, nucleus of sheath; c, nerve fibre within core; d, cells of core.

Tactile corpuscle from a duck's tongue.

Fig. 211. Tactile corpuscle from a duck's tongue, containing two tactile cells between which lies the tactile disc. (Izquier-do).

The peripheral terminals of the sensory nerves, like the other special sense organs, are usually composed of modified epithelial cells, into close relation to which the axis cylinders of nerves can be traced. They may be thus enumerated:: -

1. The Touch Corpuscles (Meissner)

The Touch Corpuscles (Meissner) are egg-shaped bodies situated in the papillae of the true skin, underlying directly the epithelial cells of the rete mucosum. They occupy almost the entire papilla. The nerve fibres seem to be twisted around the corpuscle in a spiral manner, while the axis cylinders enter the body, and the covering of the nerve becomes amalgamated with its outer wall. The touch corpuscles vary in size in different parts of the skin; usually being larger where the papillae in which they lie are well developed. The axis cylinders are said to end in swellings called tactile cells.

2. End Bulbs (Krause)

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, 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.

Drawing of termination of nerves on the surface of the rabbit's cornea.

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).

3. Touch Cells (Merkel)

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

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.