8

THE ORGANS OF TOUCH.

The Pacinian corpuscles consist of a capsule formed The of several layers, one enveloping the other. undulating nerve-fibres, after several windings, enter the capsule, which, indeed, seems to be nothing more than a much-thickened end of the outer nervecoat. These corpuscles measure from 11 to 4.5mm. They occur principally on the hands and feet, and in the flexures of the joints, but occasionally also elsewhere.

Meissner's or Wagner's corpuscles are cone-like or egg-shaped bodies, in each of which a nerve terminates, after several convolutions. They are especially numerous at the tips of the fingers, where there may be as many as a hundred in a square line. They occupy the papilla (which, however, do not always contain one), which give the surface of the hand its peculiar striped appearance. They also occur, though less numerously, elsewhere, as on the feet, breast, and lips.

NERVES OF TOUCH.

9

It appears probable, however, that these are not really the organs of touch, but rather, perhaps, guards or protectors of the true and very sensitive organs within. They are, no doubt, most numerous on the more sensitive parts of the skin, such as the hands and tongue, and the sense of touch is most acute where they occur; but they appear to be absent in some places where the sense of touch certainly exists, and they are abundant again in the foot, which, though not especially sensitive, is particularly exposed.

The sensation of pressure is intimately associated with the hairs, which no doubt serve, at any rate in some cases, for protection, but which, in Blix's opinion are in man probably all organs of touch.

We have still indeed much to learn as to the terminations of the nerves in the skin. It would seem that some are connected with cells, while others terminate in a free point. Merkel has suggested that those which end in cells are the true nerves of touch, while the free nerves record changes of temperature. Others, perhaps with more probability, have supposed that the free nerves convey merely a general and undifferentiated sensation, while those which terminate in cells give the specific impressions of pressure, heat, cold, etc., any one of which may be intensified into pain.

However, this may be, Blix * and, shortly afterwards, Goldschneidert have made the interesting discovery that we do not feel changes of pressure and of

'Exper. Beitr. zur Lösung der Frage über die Specif. Energie der Hautnerven," Zeit. für Biologie, 1885. Blix's previous papers in Upsala Läkan-forenings Förhandlingar, 1882, I have not seen.

"Monatschr. für prakt. Dermatologie." 1884. "Neue Thatsachen

ü. die Hauptsinnesnerven," Zool. Anz., 1885 und 1886.

10

SENSE OF TEMPERATURE.

temperature at the same points of the skin or by the same nerve-ends. The feeling of pressure seems to be intimately associated with the hairs, which is not the case with sensations of temperature. Even the feelings of heat and cold are also separate. These three sets of points, indeed, are so near together that the separation had hitherto not been observed, especially as they are closely intermixed. They have a tendency, however, to arrange themselves in more or less curved lines. Goldschneider experimented with a fine point, which he passed over the skin, thus testing it sometimes for pressure, sometimes with a warm point for heat, sometimes with a cold point for

Fig. 11.-Portion of the skin of the back of the hand (after Goldschneider). The centre figure represents the arrangement of the hairs; CP, the cold-points; WP. the warmth-points.

cold. Moreover, if he raised the points thus determined with a fine needle, and snipped off the fragment of the skin, he found that the resulting sensation was quite different in the three cases. If the point removed was a "pressure-point" the sensation was one for the moment of pain; while the temperature-points gave one respectively of heat or cold. The terminations of the temperature-nerves are, according to Goldschneider, much finer than those of the pressure-nerves, and they are also fewer in number. He cut out from his own skin a large number of sensitive points, but, while he found that each corresponded to a nerve-end, he has not been able to discover any difference at or in the

ORGANS OF TOUCH AMONG LOWER ANIMALS. 11

termination of the nerves corresponding to these different sensations, though it may reasonably be expected that such must exist.

The question has arisen whether there are separate nerve-endings for pain, as apart from pressure, etc.; but the observations of Blix and Goldschneider appear to show that pain arises merely from the intensification of other impressions, and that it does not reside in any special organs.

SENSE OF TOUCH AMONG THE LOWER ANIMALS.

Among the lower animals the outer skin is often very sensitive, but we know scarcely anything as to the minute structure of the organs of tactile perception. In some cases they are, no doubt, very simple; but in others it will probably be found that the apparent simplicity is due to our deficient information and means of investigation, rather than to any want of complexity in the organs themselves.

In the Cœlenterata (zoophytes, etc.) certain setæ, especially on the tentacles and near the mouth, are generally regarded as organs of touch.

In the epithelium of many of the lower animals, two forms of cells may be detected. Some unmodified, or indifferent, which form the general substance of the epithelial layer; others more or less specialized, which are seldom absolutely contiguous, but generally separated by one or more of the indifferent cells.

In other cases, nerves may end abruptly at the cuticle without the latter presenting, so far as our present means of investigation have shown, any apparent change; as, for instance, in the following

figure of a part of the skin of a small worm (Nereis).

Fig. 12.-Half of a cross section through the brain and hinder pair of eyes of Nereis cultrifera (after Carrière). 1, Hypoderm; 2, cuticle; 3, retina; 4, outer corneal cells; 5, inner corneal cells; 6, brain; 8, 8a, two places to which the brain sends large nerves (9), but where the cuticle is unaltered; g, gelatinous body.

Among the Medusa (jelly-fishes), also, the supposed tactile organs are ciliated cells (Fig. 13), which scarcely differ from the other epithelial cells, but which terminate externally in a cilia, and internally in a nervefibril.

Fig. 13.-Part of upper nerve-ring aud tactile epithelium of Lizzia (after Hertwig). a, Tactile epithelium; g, ganglionic cell; nr', upper nerve-ring.

In other cases, the tactile hairs scarcely differ from those covering the general surface. Fig. 14 represents part of the skin of a sea-anemone, the long cylinders are nematocysts, or thread-cells-elastic sacs, in the