eral localization of cutaneous sensations is really accomplished in the mind, just as, by a mental act of a different character, the impressions communicated by the eye are projected into the space about us in our thoughts, instead of being referred to the retina, or thought of as being produced in the eye itself. This power of the sensorium to localize impressions to certain points of the skin, and to project into space the stimulation caused by the light reflected from distant objects, so as to get a distinct and accurate idea of their position, is the result of experience and habit, which teach each individual that when a certain sensation is produced, it means the stimulation of a certain point of the skin, and that the objects we see are not in our eyes, where the impulses start, but at some distance from us. We learn this from a long series of unconscious experiments carried on in our early youth by movements of the eyes with coöperation of the hands. Even the sensations which arise in the various centres of the sensorium, as the result of internal or central excitations, are, from habit, attributed to external influences, and thus we have various hallucinations and delusions, such as seeing objects or hearing sounds which may only depend on the excitation of certain groups of cells in the cortex of the brain.

The sensations produced in our nerve centres as the result of the afferent impulses coming from our special sense organs give rise to a form of knowledge called perception. Each perception helps to make up our knowledge of the outer world and of ourselves. Without this power of perception we could have no notion of our own existence and no ideas of our surroundings; in fact, we should be cut off from all sources of knowledge and be idiots by deprivation of all intelligence from without.

A complete special sense apparatus may be said to be made up of the following parts:

1. A special nerve ending, only capable of being excited by a special adequate stimulus.

2. An afferent nerve to conduct the impulses from the special end organ to the nerve centre.

3. Central nerve cells, capable by specific energy of translating the nerve impulse into a sensation, which is commonly referred to some local point of the periphery.

4. Associated nerve centres, capable of perceiving the sensations, forming notions thereon, and drawing conclusions from the present and past perceptions, as to the intensity, position, quality, etc., of the external influence.

SKIN SENSATIONS.

The sensations arising from many impulses coming from the skin are grouped together under the name of the Sense of Touch. This special sense may be resolved into a number of specific sensations, each of which might be considered as a distinct kind of feeling, but is usually regarded as simply giving different qualities to the sensations excited by the skin. These sensations are: (1) Tactile Sensation, or sensation proper, by means of which we appreciate a very gentle contact, recognize the locality of stimulation, and judge of the position and form of bodies; (2) the sense of pressure; and (3) the sense of temperature.

FIG. 209.

The variety of perceptions derived from the cutaneous surface, and the large extent of surface capable of receiving impressions, make the skin the most indispensable of the special sense organs, though we value this source of our knowledge but little. If we could not place our hands as feelers on near objects to investigate their surfaces, etc., we should lose an important source of information that has contributed largely to our visual judgment. We think we know by the look of a thing what we originally learned by feeling it. If our conjunctiva did not feel, we should miss its prompt warning, and our voluntary movements could not protect our eyes from many unseen injuries that normally never trouble

Drawing from a section of injected skin, showing three papillæ, the central one containing a tactile corpuscle (a), which is connected with a medullated nerve, and those at each side are occupied by vessels. (Cudiat.)

us. If the skin were senseless, it would require constant mental effort to hold a pen, and our power of standing and walking

would be most seriously impaired. And how utterly cut off from the outer world should we be, were we incapable of feeling heat and cold.

NERVE ENDINGS.

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.

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) are egg-shaped bodies situated in the papillæ 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 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,

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 visi ble 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.