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of the pigment existing, although containing little colouring matter, I may mention, that the structure called rete mucosum exists in the skin of the white person, as well as in that of the negro, with this difference, that in the former, it contains little colouring matter, whilst in the latter, it contains a large proportion of that matter, being the seat of it, but not the colouring matter itself.
Moreover, as a counterpart of the total deficiency of colouring matter in the eyes of albino animals, with the persistence of the membrane, I may instance the pure whiteness of the skin of these animals, in which, however, I suspect the rele mucosum exists.
According to M. Desmoulins, the piginent of the eye disappears with age; the whiteness of the hairs of old people naturally strikes us as an analogous circumstance.
But to return to the structure of the membrane of the pigment. The hexagonal plates appear to be united together by means of mucous or cellular tissue, which is easily torn by a little traction, so that the fragments of the membrane always present a serrated edge, the angles being those of hexagons. Behind and around the ciliary processes, and on the posterior surface of the iris, the membrane of the pigment ceases to present the hexagonal structure, although still composed of small irregularly rounded masses of about the same size as the hexagonal plates, to which they are evidently analogous.
This change in the structure of the membrane of the pigment, which is only partial in the eyes of the mammifera, I have found to obtain in its whole extent in the eyes of those animals lower in the zoological scale which I have examined ; except in the anomalous eye of the cuttle fish, in which I found an approach to the hexagonal structure, in that part of the pigment, which is found on the posterior surface of that structure in which the crystalline lens is enchased, and which I have described, in a paper on the structure of the eye of the cuttlefish, in the Fifth Number of the Second Series of the Journal of Natural and Geographical Science, as being analogous to the ciliary processes of the eye in the vertebrate animals.
In the eye of the horse, ox, &c., where the membrane ceases to present the hexagonal structure, its texture is very loose and the colouring matter abundant.
In the human eye (one which had been macerated a long time in spirits) I found, on the contrary, the membrane stronger, and capable of being isolated in larger pieces than farther back. And by minute dissection, assisted by the microscope, it may be shown to be continued over the ciliary processes, all the projections and depressions of which it follows, so as to en
helf on the fonaked eye, "The membras little colo
tirely envelope them. Hence it is to be noted that what are commonly called the ciliary processes are composed of two parts, 1st, The vascular part, which consists of processes from the choroid. 2d, The membrane of the pigment, which moulds itself on the former, in such a manner as to appear, in the human eye, to the naked eye, like an integrant part of their proper texture, which it is not. The membrane, where it lies over the projections of the processes, contains little colouring matter, but much more in the depressions between them.
After enveloping the ciliary processes, the membrane of the pigment is continued into what is called the uvea on the posterior surface of the iris.
With regard to the chemical nature of the black particles, they chiefly consist, according to Berzelius, Mondini and Coli, of carbon and the black oxide of iron. .
Art. X.-- Account of some New Experiments on the Sen.
sibility of the Skin, by Dr WEBER, Professor of Anatomy
at Leipzig. By ALLEN THOMSON, M. D. IT is a fact well known to physiologists, that there is a considerable difficulty in pointing out with certainty, when unaided by sight, any spot on the skin that has been touched, and in distinguishing how much of the common feelings of touch is due to the sensibility of the skin, and how much is derived from the muscular sensation produced by the motion of our limbs. It is also well ascertained that some parts of the skin are better adapted than others, either from their original structure, or in consequence of their being more exercised, to convey to the mind an exact impression of the physical qualities of the bodies with which they are brought in contact. It must be allowed, however, that our knowledge respecting this part of the physiology of the sense of touch is by no means definite.
Professor Weber of Leipzig has lately performed a very simple and ingenious set of experiments which illustrate the subject of the sense of touch, and furnish us with a mode of measuring with considerable accuracy, the relative acuteness of this sense in different parts of the skin of the same or of different individuals.
These experiments consist in placing the two points of a pair of compasses at different distances from one another, and in various directions, upon different parts of the skin of an individual who is not permitted to see the bodies touching him.*
• The sharp points of the common compasses may be blunted with a little sealingWax, which will have the effect also of taking away the cold feeling of the metal
Bistriking of his es pot them originalhe Inaugural
Professor Weber thus found, that, according to the distance of the two points from one another, we may have the feeling either of one only or of two tangent points, and that the distance at which we become sensible of the double impression is in the inverse proportion to the acuteness of the sense of touch in the skin ; or, in other words, that we recognize a double impression made on very sensible parts of the skin, although the points are situated very near one another, while in those parts of the skin in which the sense of touch is obtuse, the points may be removed to a considerable distance from one another, and yet convey to us the feeling of only one impression
In August 1831, Professor Weber was so kind as to show me some of the more striking of his experiments, and at the same time presented me with an account of them originally published in detached parts as “ Annotationes" to some of the inaugural dissertations of the Leipzig University. He had also printed these annotations in a separate form, but I am not aware whether they have ever been published. To the best of my knowledge no notice of these experiments bas as yet appeared in this country, which induces me to believe that a short account of them will be acceptable to the readers of the Edin. Med. and Surg. Journal.
Professor Weber has embodied the principal results of his experiments on the varieties in the acuteness of the sense of touch of different parts of the skin in eight propositions, of which the following is an abstract.
Prop. 1. The different parts of the skin or organ of touch do not possess an equal power of distinguishing two bodies by which they are touched at the same time. The distance of the two touching bodies being known, the degree of this power may be measured; for it is ascertained that if the organ of touch does not perceive the contact of two bodies when they are near one another, it becomes sensible to the impressions of both when the distance between them is increased.
If the touching points are sufficiently distant, we not only distinguish the impressions of both, but also the direction, longitudinal or transverse in relation to the body, in which they are applied to the skin. When they are brought nearer to one another they first give the sensation of the contact of a long body, but when brought still closer together they appear as a single point upon the skin.
The ends of the fingers and the tip of the tongue have the power of distinguishing the distance of two points nearly equal, and in a much greater degree than any other part of the body. At two-fifths of a Paris line we are capable of distinguishing the longitudinal from the transverse position of the points on the tip of the tongue. At half a line two impressions are felt,
more especially when the points are made to touch at the same time the upper and lower margins of the tongue, or the dorsal and palmar sides of the fingers; but in most other parts of the body this is different; for
Prop. 2. In many parts of our bodies we perceive the distance and situation of two points touching us at the same time more distinctly when they are placed parallel to the transverse than to the longitudinal direction of the body.
This may easily be tried in the middle of the arm or forearm : here the two points may be distinguished at a distance of two inches when placed in a direction across the arm, but they appear as one at this distance, or even (in some persons) at three inches when placed longitudinally.
Under this proposition Professor Weber has placed a very long table, which may be considered as a detailed register of his experiments, and in which are exhibited the distances at which he was sensible of a single or double impression from the contact of the two points with different parts of his skin. It is difficult on a cursory view of that table to follow the general results, and on this account I have preferred giving only a shorter and more illustrative one which follows it, in which the parts of the skin are arranged according to the acuteness of their sense of touch, as measured by the smallest distance at which the horizontal or transverse position of the two points and a space between them could be distinguished.
Paris lines. Dorsal surface of the heads of the metacarpal bones, Mucous membrane of the lips near the gums, Posterior part of the zygomatic bone, . Inferior part of the forehead, Posterior part of the heel, Inferior part of the hairy occiput, Back of the hand, Neck under the lower jaw, Vertex of the head, . Patella and thigh near it, Sacrum, . Shoulder and arm near it, Gluteal region and thigh near it, Superior and inferior part of the fore-arm, Leg near the knee and foot, Dorsum of the foot near the toes, Sternum, . Spine of the back at the fifth superior vertebra, Neck near the occiput, Loins and bottom of the thorax, Middle of the back of the neck, Middle of the back, . Middle of the arm, except where the muscles swell most,
thigh, On these swellings of the muscles on the extremities, as also over the sacro lumbales, from .
36 to 42
Prop. 3. In those parts of our body in which the impressions of both points are clearly distinguished although not distant, the space between these points appears to be greater than in other parts possessing a less sensible touch.
The experiments illustrative of this are very striking. They may be best performed by drawing both the points of the compasses gently along the skin, from a sensible to a less sensible part, or vice versa ; as from the hand along the fingers, from the cheeks or ear across the lips, and towards the nose; from the jaw to the chin, from the occiput to the sacrum, with a point on each side of the median line, and from the chin to the pubis, in the same manner. In passing over the more acutely sensible parts, the points of the compasses seem to open or to recede from one another, and the reverse takes place in those regions in which the sensibility is obtuse.
Prop. 4. If the points are placed on two contiguous parts which may be moved voluntarily and independently of one another, the double impression is much more clearly perceived,