50 EPIDERMIC APPENDAGES :-NAILS, HAIR, &C.

coherent; and when thin sections are treated by a dilute solution of soda, these scales swell out again (as do also those of the cuticle) into globular cells. A new production is continually taking place in the groove of the skin in which the root of the nail is imbedded, and also from the whole of the surface beneath it; the former adds to the length of the nail; the latter to its thickness.-The structure of Hairs is essentially the same. The base of each is formed of a "bulb," which consists of a mass of epidermic cells developed from the vascular papilla at the bottom of the hair follicle (fig. 8, c); and as this narrows into the "shaft" of the hair, a difference shows itself between the cortical or outer layer, and the medullary or pith-like substance of the interior. The former, which is continuous with the outer layers of the epidermis, is composed of flattened scales, arranged in an imbricated (tile-like) manner, so that the surface of the hair is usually marked by transverse jagged lines; the latter consists of cells which frequently retain their spheroidal form, like the inner layers of the epidermis; but in the human hair these cells are elongated into fibres. It is very seldom that there is any canal in the interior of the Hair, although irregular spaces are not unfrequently left by the drying-up of the fluid contents of the cells. The structure of Quills is essentially the same as that of hairs on a large scale; and we there see the difference very distinctly marked between the cortical portion which forms the "barrel" of the quill, and the medullary portion which forms the white pith-like substance of the stem of the feather. The Scales, where they are really epidermic appendages, as is the case in serpents and lizards, are formed upon the same pattern; and we have a good example of the detachment of the entire epidermis at once (reminding us of the casting of the shell of the crab and lobster) in the "sloughing" of the snake.

39. The Mucous Membranes form a sort of internal skin, lining those cavities of the body which open on its surface; and the elements of which they are composed are essentially the same, though combined and arranged in a different manner, in accordance with their difference of function. The principal part of the thickness of every ordinary mucous membrane is made up, as in the skin, by the consolidation of areolar tissue, the fibres of which are continuous with those

MUCOUS MEMBRANES:-EPITHELIUM.

51

of the ordinary areolar tissue on which the membrane rests; this layer is copiously furnished with blood-vessels, but it is seldom supplied with many nerves. Thus the mucous membrane lining the stomach possesses in health so little sensibility, that we are not aware of the contact of the substances taken in as food, unless they are of an acrid character, or of a temperature very different from that of the body; and though the mucous membrane lining the air-passages is very susceptible of certain kinds of irritation, yet it has but little ordinary sensibility in the state of health, except near the entrance to the windpipe. The large supply of blood which these membranes receive, has reference to their active participation in the functions of secretion and absorption. One secretion is common to all, that of the mucus by which they are covered; this serves to protect them from the irritation that would otherwise be produced by the contact of solid or liquid substances, or even of air, with their free surfaces; and we see the results of its deficiency, in the inflammation which attacks the membrane, sometimes proceeding to its entire destruction, when from any cause the secretion is checked, as it sometimes is by injuries of the nerves supplying the part.

40. In every mucous membrane, as in the skin, the fibrous texture is bounded on the free surface by basement-membrane, beyond which no blood-vessels pass. And the surface of the basement-membrane is covered by cells, arranged either in a single layer or in multiple layers, constituting the Epithelium. This, although answering to the Epidermis in structure and position, has a very different character; for its cells neither dry up nor become horny; nor do they adhere in such a manner as to form a continuous membrane, except in the interior of the mouth and oesophagus (gullet), where the epithelium is endowed with somewhat of the firmness of cuticle, in order to resist the abrading contact of hard substances. The epithelium cells of mucous membranes are commonly somewhat flattened; but in some situations, as on the villi of the intestinal canal (fig. 9, d), they have more of a cylindrical, or rather conical shape, their smaller extremities being in contact with the basement-membrane. The epithelial cells are frequently cast off, like the epidermic, especially from the parts that are most concerned in secretion;

52

MUCOUS MEMBRANES:-EPITHELIUM.

and they are as continually replaced by newly-formed cells, which are produced on the surface of the basement-membrane, at the expense of the fluid that transudes through it from the blood-vessels copiously distributed to its under surface.

41. Mucous membrane may either exist in the condition of a simple expanded surface, or may have a much more complex arrangement, by which its surface is greatly increased. The simple mucous membrane, such as that which lines the nose and air-passages, is found, for the most part, where no absorption has to be performed, and where only a moderate amount of secretion is necessary. But where it is to absorb as well as to secrete, it is usually involuted or folded upon itself, in such a manner as to form a series of little projections, and also a number of minute pits (fig. 9). These pro

Fig. 9.-DIAGRAM REPRESENTING THE MUCOUS MEMBRANE OF THE
INTESTINAL CANAL.

a a, absorbent vessels; bb, basement membrane; c c, epithelium-cells of level surface of membrane; dd, cylindrical epithelium-cells of villus; e e, secreting cells of follicle.

jections sometimes have the form of long folds; in other instances they are narrow filaments, crowded together so as almost to resemble the pile of velvet.. In either case, the absorbent surface is vastly increased; but chiefly so by these filaments, which are termed villi, and act as so many little rootlets. On the other hand, it is in the pits or follicles, that the production of the fluid which is to be separated or secreted from the blood, chiefly takes place.-Not only are the flat expanded surfaces of the mucous membrane covered with epithelium cells, but the villi also are sheathed by them; and the secreting follicles are lined by the same.

STRUCTURE OF GLANDS.SEROUS MEMBRANES.

53

The cells covering the villi (fig. 9, d) perform the important function of selecting and absorbing certain nutritious elements of the food, which they communicate to the absorbent vessels in the interior of the villi. On the other hand, the epithelium-cells of the follicles (e) seem to be the real agents in the secreting process; drawing from the blood, as materials for their own growth, certain elements contained in it; and falling off, when mature, so as to discharge these substances as the product of secretion, giving place to a fresh crop or generation of cells, which go through a series of changes precisely similar to the preceding.

42. Now these follicles are the simplest types or examples of all the Glandular structures, by which certain products are separated from the blood, some to be cast forth from the body as unfit to be retained in it, and some to answer particular purposes in the system. In all of them the structure ultimately consists of such follicles, sometimes swollen into rounded vesicles, and sometimes extended into long and narrow tubes. Each follicle, vesicle, or tube, is composed of a layer of basement-membrane, lined with epithelium-cells, and surrounded on the outside with minutely distributed blood-vessels; and it seems to be by the peculiar powers of these cells, that the products of the secreting action, whether bile, saliva, fatty matter, or gastric fluid, are formed (see Chap. vII.).—Hence we see that the act of Secretion is, in animals as in plants, really performed by cells. It is necessary to bear in mind, however, that a simple transudation of the watery parts of the blood may take place without any proper secreting action, in the dead as in the living body; it is in this manner that the serous fluid of areolar tissue and serous membrane is poured out, and that the watery portion of the urine is separated.

43. The Serous Membranes which line the closed cavities of the body, though composed of the same elements as the skin and mucous membranes, have a much simpler structure, and can scarcely be said to minister directly to any important vital function. The tissue of which Serous membrane is principally composed, scarcely differs, except in its greater density, from the laxer areolar tissue whereby the membrane is attached to the walls which it covers like plaster; it is but sparingly supplied either with blood-vessels or absorbents; and

54

ARRANGEMENT OF SEROUS MEMBRANES.

it contains very few nerves. The smooth surface of the membrane forms one unbroken plane, being neither raised into villi, nor depressed into follicles; and its basement-membrane is covered with a single layer of flat epithelium-cells, which

Fig. 10. PAVEMENT EPITHE

MEMBRANE.

are closely applied to it and to each other, like the pieces of a pavement (fig. 10). It is with such a membrane that every one of those great cavities is lined, which contains important viscera; and it is also continued on to the outer surface of these viscera, so as to afford them an external coating over every part save that by which they are attached. Thus the heart is suspended freely, by the large vessels proceeding from LIUM CELLS OF SEROUS its summit, within a bag or sac of fibrous membrane peculiar to itself, which is termed the pericardium. The cavity of this bag is completely lined by the serous membrane (fig. 11, p'), which closely embraces the vessels, and which then bends down over the surface of the heart, so as to enclose it in the envelope p. Hence it will be seen that this membrane, whilst including the heart, and allowing it to communicate with its vessels, P forms a completely shut sac;

and it may be likened to a common double cotton or woollen night-cap, which has a similar cavity between its two layers, the head being really on the outside of this, whilst seeming to be within the envelope. The two layers of the pericardium, though separated in the diagram for the sake of distinctness, are really in mutual contact, save when separated by the interposition of fluid poured out in disease. Each of the lungs, in like manner, is suspended in a closed sac of its own, termed the pleura; and the surface of the lung is covered by a serous membrane, which is reflected over the wall of the pleural cavity.

Fig. 11.-DIAGRAM OF THE PERICARDIUM.

a a, auricles; vv, ventricles; b, pulmonary artery; c, aorta; pp', pericardium.