through by transudation; but that the albuminous matter will be for the most part kept back, so that only a very small proportion of it is to be found in the transuded liquid. This appears to be a sufficient explanation of the presence of a weak serous fluid in the cavities of areolar tissue; and there is not any necessity, therefore, to imagine the existence of a secreting power, either in the areolar tissue itself, or in the walls of the capillaries which traverse it. When there is a want of firmness or tone in the walls of the vessels, producing (as we shall hereafter see, § 609) an increased pressure of the contained fluid on their walls, and diminished resistance, the watery part of the blood will have an unusual tendency to transudation: and we accordingly find that it then distends the areolæ, and produces dropsy. The physical arrangement of the parts of the tissue is so much altered, that its elasticity is impaired; and it consequently pits on pressure, that is, when pressure has made an indentation in the surface, this is not immediately filled up when the pressure is withdrawn, but a pit remains for some seconds or even minutes. The free communication which exists among the interstices, is shown by the influence of gravity upon the seat of the dropsical effusion; this always having the greatest tendency to manifest itself in the most depending parts,-a result, however, which is also due to the increased delay that takes place in the circulation in such parts, when the vessels are deficient in tone. The freedom of communication is still more shown, however, by the fact, that either air or water may be made to pass by a moderate continued pressure, into almost every part of the body containing Areolar tissue; although introduced at only a single point. In this manner it is the habit of butchers to inflate veal and impostors have thus blown up the scalps and faces of their children, in order to excite commiseration. The whole body has been thus distended with air by emphysema in the lung; the air having escaped from the air-cells into the surrounding areolar tissue, and thence, by continuity of this tissue with that of the body in general at the root or apex of the lungs, into the entire fabric.

Fig. 8.

C

197. The structure of the Serous and Synovial Membranes is essentially the same with that of Areolar tissue. It is the peculiar character of these membranes to form closed bags or sacs, having a very smooth and glistening inner surface, and containing a fluid more or less allied in composition to the serum of the blood. The disposition of the Synovial membranes may be understood by studying one of the simpler forms of the joints, such as is represented in the accompanying diagram; but although Ideal section of a Joint; originally continuous over the surfaces of the Articu- a, a, the extremities of lar Cartilages, the Synovial membrane does not con- b, b, the layers of carti tinue to be distinctly traceable after the joint has come e, c, e, c. synovial mem into play, and its vessels retreat from the portion over branes covering the artiwhich the two surfaces are exposed to friction, but passing from one to the from a circle round its margin, from which the Cartilage is nourished (§ 278). The arrangement of the Serous membranes

the two articulated bones;

lage which cover them:

culated surfaces, and

other.

is usually much more complicated. These line the three great cavities of the body, the head, chest, and abdomen,-together with their subdivisions; enveloping the viscera. which these contain, so as to afford them an external coating over every part save that by which they are suspended; and being then reflected over the interior of the cavity, so as to form a shut sac intervening between its outer walls and its contents. The chief purpose of this appears to be, to facilitate the movements of the contained organs, by forming smooth surfaces which shall freely glide over each other; this is evidently of great importance, where such constantly moving organs as the heart and lungs are concerned.

198. The free or unattached surface of these membranes is covered with a layer of cells; but these constitute a distinct tissue, the Epithelium, of which an account will be given hereafter. The epithelium lies upon a continuous sheet of membrane, of extreme delicacy, in which no definite structure can be discovered; the nature of this, which is called the basement or primary membrane, will be presently considered (§ 206). Beneath this is a layer of condensed Areolar tissue, which constitutes the chief thickness of the serous membrane, and confers upon it its strength and elasticity; this gradually passes into that laxer variety, by which the membrane is attached to the parts it lines, and which is commonly known as the sub-serous tissue. The yellow fibrous element enters largely into the composition of the membrane itself; and its filaments interlace in a beautiful network, which confers upon it equal elasticity in every direction. The membrane is traversed by blood-vessels, nerves, and lymphatics, in varying proportions; some of the synovial membranes, especially that of the knee-joint, are furnished with little fringe-like projections, which are extremely vascular, and which seem especially concerned in the secretion of the synovial fluid. The fluid of the serous cavities is so nearly the same as the serum of the blood, that the simple act of transudation is sufficient to account for its presence in their sacs; on the other hand, that of the Synovial capsules, and of the Bursa Mucose which resemble them, may be considered as serum with from 6 to 8 per cent. of additional albumen.

199. The elements of Areolar tissue enter largely also into two other textures, which perform a most important share in both the Organic and the Animal functions ;-namely, the Mucous Membranes and the Skin. These textures are continuous with each other; and may, in fact, be considered as one and the same, modified in its different parts according to the function it is destined to perform. Thus it is everywhere extremely vascular; but the supply of blood in the Skin is chiefly destined for the nervous system, and is necessary to the act of sensation; whilst that of the internal skin or Mucous Membrane is rather subservient to the processes of absorption and secretion. This tissue is continued inwards from the external surface of the body, by the several orifices and outlets of its cavities; and it is further continued most extensively from its primary internal prolongations, into the inmost recesses of the glandular

structures.

200. Thus the Gastro-intestinal mucous membrane commences at the mouth, and lines the whole alimentary canal from the mouth to the anus, where it again becomes continuous with the skin; and it sends off as

branches, the membranous linings of the ducts of the salivary glands, pancreas, and liver; these membranes proceed into all the subdivisions of the ducts, and line the ultimate follicles or cæca in which they terminate. Again, the Bronchio-pulmonary mucous membrane commences at the nose, and passes along the air-passages, down the trachea, through the bronchi and their subdivisions, to line the ultimate air-cells of the lungs; communicating in its course with the gastro-intestinal. Another mucous membrane of small extent commences at the puncta lachrymalia, lines the lachrymal sac and the nasal duct, and becomes continuous with the preceding. Another, which may be considered a kind of offset from either of the first two, passes up from the pharynx along the Eustachian tube, and lines the cavity of the tympanum.

201. Near the opposite termination of the alimentary canal, moreover, we have the Genito-urinary mucous membranes; these commence in the male by a single external orifice, that of the urethra ;-passing backwards along the urethra, the genital division is given off, to line the seminal ducts, the vesiculæ seminales, the vasa deferentia, and the secreting tubili of the testis; another division proceeds along the ducts of the prostrate gland, to line its ultimate follicles, and another along the ducts of Cowper's glands; whilst the urinary division lines the bladder, passes up along the ureters to the kidney, and then becomes continuous with the membrane of the tubuli uriniferi. In the female, the urinary division commences at once from the vulva; whilst the genital passes along the vagina into the uterus, and thence along the Fallopian tubes to their fimbriated extremities, where it becomes continuous with the serous lining of the abdominal cavity, the peritoneum.

202. Besides the glandular prolongations here enumerated, there are many others, both from the internal and external surface. Thus we have the Mammary mucous membrane, commencing from the orifices of the lactiferous ducts, passing inwards to line their subdivisions, and forming the walls of the ultimate follicles. In the same manner the Lachrymal mucous membrane is prolonged from the conjunctival mucous membrane, which covers the eye and lines the eyelids, and which is continuous with the skin at their edges. There are several minute glands, again, in the substance of the skin, and in the walls of the alimentary canal, which need not be here enumerated; but which contribute immensely to the extension of the surface of the mucous membrane, a prolongation of this being the essential constituent in every one. In their simplest form, these glandulæ are nothing more than little pits or depressions of the surface; these are found both in the Skin and Mucous membrane, and are particularly destined for the production of their protective secretions, hereafter to be described.

203. We have seen, then, that the essential character of the Mucous membranes, as regards their arrangement, is altogether different from that of the serous and synovial membranes. For whilst the latter form shut sacs, the contents of which are destined to undergo little change, the former constitute the walls of tubes or cavities, in which constant change is taking place, and which have free outward communications. Thus in the gastro-intestinal mucous membrane, we have an inlet for the reception of the food, and a cavity for its solution, the walls of which

are endowed in a remarkable degree with absorbing power, whilst they are also furnished with numerous glandulæ, which pour the solvent fluid into the cavity. On the other hand, it has an outlet, through which the indigestible residuum is cast forth, together with the excretions from the various glands that pour their products into the alimentary tube. In the bronchio-pulmonary apparatus, the same outlet serves for the introduction and for the expulsion of the air; and here, too, is continual change. In other cases, there is but a single outlet; and the change is of a simpler character, consisting merely in the expulsion of the matters eliminated from the blood by the agency of the glands. Now it is, as we shall see hereafter, in the digestion and absorption of food, on the one hand, and in the rejection of effete matters on the other, that the commencement and termination of the nutrient processes consist; and these operations are performed by the system of Mucous membranes, including in that general term the Skin, which is an important organ of excretion, besides serving as the medium through which sensory impressions of a general character are received by the Nervous system.

204. The Mucous Membrane may be said, like the Serous, to consist of three chief parts;-the epithelium or epidermis covering its free surface;-the subjacent basement-membrane;-and the areolar tissue, with its vessels, nerves, &c., which forms the thickness of the membrane, and connects it with the subjacent parts. The Epidermis and Epithelium alike consist of cells; but the function of the former (which consists of several layers, of which the outer are dry and horny) is simply protection to the delicate organs beneath; whilst that of the latter is essentially connected with the process of Secretion, as will be shown hereafter. The basement-membrane resembles that of the serous membranes; but its separate existence is unusually evident in some parts where it exists alone, as in the tubuli uriniferi of the kidney; whilst it can with difficulty be demonstrated in others, as the skin. The Areolar tissue of Mucous membranes usually makes up the greatest part of their thickness; and it is so distinct from that of the layers beneath, constituting the sub-mucous tissue, as to be readily separable from them. It differs not in any important particular, however, from the same tissue elsewhere; and the white and fibrous elements may be detected in it in varying proportions, in different parts,-the latter being especially abundant in the skin and lungs, which owe to it their peculiar elasticity. Hence the Mucous membranes yield Gelatine in abundance, on being boiled. The skin also appears to contain some of the non-striated Muscular fibre (§ 337), in varying proportions in its different parts.

205. The relative amount of Blood-vessels, Nerves, and Lymphatics, as already mentioned, is subject to great variation, according to the part of the system examined. The first, however, are most constantly abundant, being required in the Skin for sensation (Fig. 9), and in the Mucous membranes for absorption and secretion (Figs. 10, 11, 12). In fact we might say of many of the mucous membranes, especially those of the glands, that their whole purpose is to give support to the secreting cells, and to convey blood-vessels into their immediate neighborhood, whence these cells may obtain materials for their development. The Skin is the only part of the whole system which is largely supplied with

Nerves (Fig. 13), except the Conjunctival membrane and the Mucous membrane of the mouth and nose; hence the sensibility of the internal

mucous membrane is usually low, although its importance in the organic functions is so great. The Skin is copiously supplied with Lymphatics;

and the first part of the alimentary canal with Lacteals; some of the glandular organs are also largely supplied with Lymphatics.-The

Distribution of the tactile nerves at the extremity of the human thumb, as
seen in a thin perpendicular section of the skin.

Areolar tissue, whether existing separately, or forming a part of the Serous and Mucous Membranes, is capable of being very quickly and completely regenerated; indeed, we often find that losses of substance in other tissues are replaced by means of it.

3. Of the Basement or Primary Membrane.

206. In many parts of the Animal body, we meet with membranous expansions of extreme delicacy and transparency, in which no definite