The property of determining the point touched is called the skin's sense of locality, which is a factor of the tactile sense. The perception of contact is not sufficient to explain this phenomenon; the process consists much more in the localisation of the contact perceived. This power is an important psychical action, which evidently takes place in the brain, and can only be explained by the supposition, that in our imagination a picture, as it were, of our skin's surface exists, in which we seek for the position of the spot where contact has taken place, and which we find with greater or less certainty. The question, what is the relation between the skin's surface and the imaginative faculty, i.e. the brain, in which the faculty is situated, is sufficiently answered by the physiology of the nerves.

A great number of sensory nerves radiate from the brain and the spinal cord to the skin; they all consist of a very great number of fibres which separate from each other near the skin and here terminate in a peculiar

manner.

The skin itself consists of three layers (fig. 1).1 Upon the cellular tissue d f under the skin, which sometimes is very rich in fat, lies the first skin, the dermis (from c to b), which is of a tolerably compact texture, and by tanning is converted into leather. Its surface consists of a greater or less number of cylindrical or conical protuberances, e, which are called papilla. Upon the dermis lies the mucous layer, b, which consists of a great number of small microscopic cells completely filling the depressions between the papillæ of the dermis. Lastly, the outer layer is the cuticle or epidermis, a, which forms

1 Kölliker, Gewebelehrer.'

a compact firm skin, but consists of an intergrowth of cells which are filled with a solid horny substance. The blood-vessels and nerves extend only as far as the surface of the dermis, and to its papillæ; the mucous layer and the epidermis are completely free from blood and nerves. In the figure are seen also the sweat-glands, g, in the skin, i sweat glands.

cuticle, or
epidermis.

16 mucous layer. dermis

papillae

cellular home

Fig. 1.

whose ducts, h, penetrate to the surface of the skin, i. The nerves of the skin which terminate in single fibres only extend to the dermis, and here they are observed to end in a peculiar manner in the papillæ. Many of them contain, for instance, an egg-shaped particle, ỏ (fig. 2), which a nerve fibre, c, enters and in which it is lost after

C

b

several convolutions, dd, round it. They are called tactile corpuscles, and there can be no doubt that they act as

A pafilla

C. herve fibre corpuscle.

Fig. 2.

the instrument of the sensation of touch. They are not found in the same numbers in all parts of the skin, occurring in the greatest number in those parts where the sensibility is more acute, and more sparingly where it is weaker.

They are extraordinarily numerous at the tips of the fingers, where, in the space of a square

line, about 100 may be counted,

and they are tolerably numerous over the whole surface of the hand, but occur in much smaller numbers on the backs of the hands. On the palm of the hand also the papillæ, which, however, do not all contain a tactile corpuscle, occur in great numbers and are arranged in regular rows. This gives the peculiar striped appearance of the skin which is perceived on the surface of the hand. The nerves of the skin are observed to possess another terminal apparatus, similar to that of the tactile corpuscles, namely, long globules (pacinian bodies), in the hollows of which the nerve fibres terminate. In short, in the entire surface of the skin there exist terminal apparatus of a peculiar kind for the sensory nerves, and if we wish to follow the action of sensation further physiologically, we must start with the excitement of a nerve fibre which ends in a definite part of the skin, and follow the course of the excitement to the brain.

The course of the nerve between brain and skin along

which the excitement passes can be followed anatomically with a certain degree of exactness. A nervous fibre which ends in the skin forms as far as its union with the spinal cord or brain, a long, fine, continuous thread. The fibres which terminate in the skin very soon

unite in small branches, and finally in thick nerve-trunks, Physi

before they enter the central organ of the nervous system, but in no case do two nervous fibres coalesce in these nerve-branches. We may, therefore, assume that every part of the skin is provided with isolated connections with the centre of the nervous system, which are united there just as telegraph lines unite at a terminus.

The physiology of the nerves, in which great advance has been made by the study of the action of motory nerves, throws light upon the course of nerve irritation, and leads to the result that the irritation of a fibre passes through its entire length without being communicated to the adjoining fibres. In a telegraph wire, of course, the electric current must by complete isolation be prevented from passing along any other wire, if the intelligence is to reach the station intended. The nervefibres on the contrary, in which we have to do with an action entirely different from, and much less rapid than electricity, do not require such isolation, and although they are closely packed together to the number of several thousand, yet they allow of no transfer of the irritation from one to the other.

When, by touching a part of the skin, we irritate the nerve which ends there, the irritation, insulated in the nerve, passes along it to its origin in the nervous centre. As soon as the irritation has arrived there, the action of sensation takes place; we know nothing of what this

A

consists, except that it can only take place in the nervous centre. This sensation is immediately connected with the consciousness that the irritation has been experienced by a certain definite part of the skin; our imagination even places the entire action of sensation in the skin itself, although it cannot take place in the skin, but only in the brain. It is absolutely impossible for us to separate the sensation itself from the part irritated. In our imagination the locality of the irritation and that of the perception coincide exactly, and we are first taught by physiological experiments and the diseases of the nerves that this is, in reality, not the fact. The fact that the division of a nerve deprives a part of the body of sensation, shows us that the sensation itself cannot take place in the limbs, nor in the nerve itself, and that a nerve can only produce it when its connection with the brain and the spinal cord is intact.

The law is equally applicable to all sensory nerves, that we refer the sensation which they produce to the organs at the terminations of the nerves, to the point where the cause of the irritation acts. This is the case not only with the sensory nerves of the skin, but to a much greater extent with the optic nerve, the irritation of which takes place in the retina of the eye. The sensation of sight can only take place, however, in the brain, for it is lost as soon as the optic nerve is divided, and yet we transfer the object seen to the external world surrounding us. This fact, which is called the law of eccentric sensation, can only be explained by the assumption that our imaginative faculty is entirely formed by experience. We know from experience what our sensation is as soon as the contact with the skin has

+ sensuous imagination.