202

The Skin.

[Book IX. fenfibility, occafion much uneafinefs from the friction to which the furface of the body is neceffarily expofed. The epidermis confifts of a mucous fubftance, which is fituated next the true skin, and a dry, transparent, and in fome measure horny fubftance, which is external.

The mucous fubftance, called corpus mucofum, or rete Malpighianum, is of a consistence between that of a folid and a fluid, and is often treated of by anatomists as a diftinct covering cf the body. The colour of it varies according to the complexion. In fair people it is white, in brown people of a dusky hue, and in the Africans black. In the latter it is alfo more folid, and can be feparated from the external part of the epidermis, which cannot be effected in Europeans. By friction, the epidermis gains very much in thickness, as may be obferved in the hands of labouring people, and in the foles of the feet of those much accustomed to walking, Corns, which are nothing but hardened epidermis, are the confequence of the preffure and friction of tight fhoes *.

The epidermis is not furnished either with nerves, or blood-veffels, and is therefore infenfible. The Abbè Fontana fubmitted fome very minute portions. of the epidermis, taken from his hand, to examination by a microscope which magnified seven hundred diameters, The epidermis appeared to be composed of winding cylinders, which approached each other, and retreated with much regularity and order; fmalk globules alfo were in parts perceptible. When the

*The cure of, thefe difagreeable excrefcences is very obvious. from this account; nothing is indeed required for this purpose, but to cover them with any soft adhesive fubftance, which will protect them from friction, when they will naturally decay, and in time come off spontaneously.

portion

portion of epidermis was covered with water, it appeared more tranfparent, and the cylinders and globules were seen more diftinctly. He could obferve nothing, however, like perforations or holes in the epidermis, and therefore doubts of their existence. It feems probable, the Abbè Fontana adds, that the lymphatics, which le Pere della Torre pretends to have feen in the epidermis, were nothing but these winding cylinders.

We must believe, however, from the quantity of fenfible and infenfible perfpiration, especially in warm climates, where, according to Sanctorius, who made his experiments in Italy, it amounts to five-eighths of the food taken in, that there are perforations in the epidermis for the paffage of exhalent arteries, It may be alfo added, that the appearances exhibited by objects fubmitted to microscopes of high powers are never much to be depended on, and have given rise to numerous deceptions,

Immediately below the fkin of quadrupeds, except those of the porcine (fwine) fpecies, lies a thin fleshy expanfion, called panniculus carnofus, covering the greater part of the body, and furrounding the other muscles. In man there is nothing similar to this, excepting the platifma myoides, or the occipitofrontalis muscle. The use of this thin muscular expanfion is to wrinkle and move the skin in order to hake off duft, infects, &c,

I 204 1

[Book IX.

CHAP. XVI.

THE HAIR AND THE NAILS.

Opinions of Anatomists with respect to the Nature of the Hair, Nails, &c.-Hair originates from the Cellular Subftance.-Fontana's Obfervations on Hair-The Nails.-The Horns, Hoofs, and Claves of

Animals.

M

ANY anatomifts chufe to call the hair, the nails, and the horns of animals, productions of the epidermis; by Malpighi and Rush the hairs were fuppofed to be continuations from the nerves; neither of which opinions, however, feems to be fufficiently proved, though the former appears by far the more probable. The hairs are distributed more or less remarkably over the whole body, except on the palms of the hands, and foles of the feet. They rife each of them from a feparate oval bulb placed beneath the true fkin, and lodged in the cellular fubftance, and they are furrounded by a fheath, which rifes with them as far as the furface of the body.

The Abbè Fontana took a hair, which he cleanfed by repeatedly drawing it through a piece of fine linen dipt in water; he examined it with lenfes of different powers, from fome which magnified 400, to others which magnified 700 diameters, and the ap pearances, he informs us, were uniformly the fame. The hair in general appeared of the colour of tranfparent amber; towards the center, however, of it, there was an obfcure line, which was broken at one part. It appeared woven, and formed by, or covered with, twisting cylinders, interrupted at places, and winding like the inteftines of animals. Among the

winding

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1

winding cylinders there appeared minute globules of the fame diameter with the cylinders. Having crushed the hair at one of its extremities, it appeared as if formed of many irregular polished trunks, which were compofed of bundles of very fmall winding cylinders, with fome globules fcattered on the cylinders themselves.

The nails are horny infenfible bodies, formed of thin lamellæ or plates. They rife by a fquare origin from the last joints of the fingers and toes, and are hard where they are expofed to the air, but foft near their roots. The structure of the horns, hoofs, and claws of animals is very fimilar to that of our nails. A minute portion of a finger nail being fubmitted to the microscope, exhibited the fame appearance as the epidermis. Both the nails and hair grow entirely from below, by a regular propulsion from their roots.

CHAP. XVII.

THE CAVITY OF THE ABDOMEN.

Contents of the Abdomen.—Parts involved by the Peritoneum.Parts not involved by it.-The Peritoneum.-The Mefentery.-The Omentum.-Different in Man and Quadrupeds.

1

HIS cavity is bounded above by the diaphragm,

below by the bones of the pelvis, at the fides by various muscles and the falle ribs, before by the muscles of the abdomen, and behind by the vertebræ of the loins and the os facrum. Strictly speaking, however, no part is faid to be within the cavity of the abdomen, which is not involved in a thin transparent membrane, called the peritoneum, of which a more particular description will presently be fubmitted to

the reader.

The parts which are involved in the peritoneum are, the mefentery, the omentum or caul, the ftomach, the small and great inteftines, the lacteal veffels, the pancreas, the fpleen, and the liver.

The organs which are not involved in the peritoheum, but are placed behind it, are the kidneys, the ureters, the receptacle of the chyle, the aorta, and the

vena cava.

The upper part of the bladder is involved in the peritoneum, the lower is placed without it.

The peritoneum is to be confidered as a membrane forming an internal covering to the parts which are the boundaries of the abdomen, and at the fame time doubled back on itself in fuch a manner as to form the external covering of the abdominal vifcera.

The internal furface of the peritoneum is fmooth,