tissue (fig. 440; fig. 443, r), the fibres of which are for the most part covered and concealed in the natural condition by ramified cells, and is filled with fluid. lymph, containing many lymph-corpuscles, which may be washed out from sections of the gland, so as to show the sinus, while the firmer gland-pulp, which the sinus surrounds, keeps its place. The proper glandular substance is also pervaded and

Fig. 443.-RETICULUM FROM THE MEDULLARY PART OF A LYMPHATIC GLAND. (E. A. S.) tr, end of a trabecula of fibrous tissue; r, r, open reticulum of the lymph-path, continuous with the fibrils of the trabecula; r', r', denser reticulum of the medullary lymphoid cords. The cells of the tissue are not represented, the figure being taken from a preparation in which only the connective tissue fibrils and the reticulum are stained.

supported by fine retiform tissue (figs. 443, r', 445, a), communicating with that of the surrounding lymph-sinus, but marked off from it by somewhat closer reticulation at their mntual boundary, not so close, however, as to prevent fluids, or even

Fig. 444.-END OF A FIBROUS TRABECULA

FROM THE SAME PREPARATION, SHOWING
THE CONTINUITY OF THE CONNECTIVE
TISSUE FIBRILS WITH THE RETICULUM:

HIGHLY MAGNIFIED. (E. A. S.)

tr, trabecula; r, reticulum.

corpuscles, from passing from the one to the other. The gland-pulp is otherwise made up of densely packed lymphoid cells, occupying the interstices of its supporting retiform tissue, and usually exhibiting, especially at its peripheral parts,

abundant evidence of the process of division and multiplication by karyokinesis. This lymphoid tissue is traversed by a network of capillary blood-vessels (fig. 445,d,d), which run throughout the proper glandular pulp, both cortical and medullary, but do not pass into the surrounding lymph-sinus. The lymphoid cells of the glandular pulp are similar in their general appearance to white blood- or lymph-corpuscles, except that their nucleus is relatively larger, and their protoplasm less in amount.

The ramified cells which cover the retiform tissue of the lymph-sinus, often contain a considerable number of pigment-granules, especially in the medulla of the gland. The trabeculæ themselves have a covering of flattened cells, which on the side turned towards the lymph-channel are provided with processes to anastomose with those covering the retiform tissue. The inner surface of the capsule is also lined with flattened cells, which are continuous at the entrance and exit of the lymphatics with the endothelium of those vessels.

Arteries enter and veins leave the gland at the hilum, surrounded, in some

glands, as already said, with a dense investment of connective tissue. The arterial branches go in part directly to the glandular substance, but partly run along the The former end in the glandular capillary network above-mentioned, from which the veins begin, and tend to the hilum alongside the arteries. The branches which run along the trabeculæ are partly conducted to the coat of the gland to be there distributed; but most of their branches pass to the glandular substance, the connective tissue of the trabeculae which ensheaths them passing gradually into the lymphoid tissue of the pulp, so that this at first appears as a

Fig. 445. SECTION OF THE MEDULLARY SUBSTANCE OF A LYMPHATIC GLAND (OX). 300 DIAMETERS. (Recklinghausen.)

a, a, a, follicular or lymphoid cords; b, b, trabeculæ ; c, lymph-sinus; d, d, blood-vessels.

sheath to the arterial branch (as in the spleen). The latter soon, however, breaks up into capillaries which ramify in the gland-pulp, supported by its pervading retiform tissue, which forms an additional adventitious coat around the minute vessels.

As to the lymphatics of the gland, the afferent vessels, after branching out upon and in the tissue of the capsule, send their branches through it to open into the lymph-sinuses of the cortex, and the efferent lymphatics begin by fine branches leading from the lymph-sinuses of the medullary part, and forming at the hilum a dense plexus of tortuous and varicose-looking vessels, from which branches proceed to join the larger efferent trunks. The lymph-sinus, therefore, forms a path for the passage of the lymph, interposed between the afferent and efferent lymphatics, communicating with both and maintaining the continuity of the lymph-stream. The afferent and efferent vessels, where they open into the lymph-sinus, lay aside all their coats, except the epithelial lining, which is continued over the trabeculæ and the interior of the capsule.

The chief differences of structure which are seen in lymphatic glands depend upon the relative amount and nature of the framework. Thus whereas in some animals both the capsule and the trabeculæ are strong and muscular, in others they are less developed and contain but little plain muscular tissue. In some animals

the trabeculæ are almost or entirely absent, and the interior of the gland then looks in section like a continuous mass of lymphoid tissue, traversed by lymphatic channels.

It is not unreasonable to presume that, in the proper glandular substance, there is a continual production of lymph-corpuscles, which pass into the lymph-sinus, and that fresh corpuscles are thus added to the lymph as it traverses the gland. This view is supported by the fact, that the corpuscles are found to be multiplying by karyokinesis within the pulp, and are more abundant in the lymph or chyle after it has passed through the glands.

Other organs composed of lymphoid tissue.-Bodies which are similar in structure to lymphatic glands in so far that they are composed of a delicate retiform tissue, the interstices of which are closely packed with lymphoid cells, and are in intimate relation with the lymphatic vessels of the part, occur in many places. Thus, in the serous membranes, rounded nodules are here and there met with, which, as Klein has shown, are developed either around or at one side of an enlarged lymphatic (perilymphangial nodule, fig. 446, A), or in some cases even within the vessel (endolymphangial nodule, fig. 446, B). The retiform tissue which constitutes the framework of the nodule is connected with the wall of the lymphatic, and lymphoid cells accumulate in the interstices of the retiform tissue, where they multiply and ultimately form a dense mass of lymphoid tissue.

The endolymphangial nodules, although small and simple in structure, closely recall the structure of one of the cortical nodules of a lymphatic gland; for a path

Fig. 446.-DEVELOPING LYMPHATIC (LYMPHANGIAL) NODULES, FROM THE OMENTUM OF A GUINEA-PIG.

(Klein.)

A, perilymphatic nodule; a, lymphatic vessel; c, part of its epithelial wall, seen in optical section; e, lymph-corpuscles within the vessel; b, lymphoid tissue of the nodule; d, blood-capillaries. B, endolymphatic nodule; a, vein; b, artery; c, capillaries; d, a lymphatic vessel, in which this whole system of blood-vessels is inclosed; e, lymphoid tissue within the lymphatic vessel; f, wall of the lymphatic in optical section.

or channel for the passage of lymph is left between the central accumulation of lymphoid tissue and the wall of the vessel, this path being bridged across by retiform tissue and branched cells; and along it the lymph must pass very slowly, and come into intimate relation with the tissue of the nodule. In other cases the lymphoid tissue of the serous membranes is less circumscribed, occurring in the form of ill-defined patches or elongated tracts, which lie along the course of the small arteries and veins, receiving from the latter branches which form a capillary network within the tissue.

In mucous membranes, especially that lining the alimentary canal, conspicuous lymphatic nodules are met with in various parts, and here they have been long known. They occur either singly, as in the so-called solitary glands of the intestine, or collected into groups as in the agminated glands or patches of Peyer, or into thick masses as in the tonsils. In most of these cases the nodules are spherical or dome-shaped condensations of the lymphoid tissue which occurs in the substance of the mucous membrane, on the surface of which they may cause a distinct prominence; they are usually found to be in close relation with the lymphatics of the membrane, being either partially surrounded by a large sinus-like lymphatic, or encircled by a plexus of lymphatic vessels. In the mucous membrane of the bronchial tubes lymphoid nodules are met with which are quite similar to those of the alimentary mucous membrane. In the spleen, tracts of lymphoid tissue, with lymphatics in connexion with them, ensheath the smaller arteries and are dilated at certain points into distinct nodules which have here been long known as the Malpighian corpuscles of the spleen. Lastly the thymus gland seems to be chiefly composed in the young subject of lymphoid tissue having a more or less nodulated arrangement, although in the adult this organ is usually found to have become transformed into adipose tissue.

It has been shown by Flemming that the nodular formations of lymphoid tissueare in all cases due to the rapid multiplication of cells which is occurring at the spots where they are found; that they represent, in short, foci of multiplication.

The further description of the lymphoid structures will be deferred until the several organs where they occur are systematically treated of.

SEROUS MEMBRANES.

The serous membranes are so named from the apparent nature of the fluid with which their surface is moistened. They lie in cavities of the body which have no obvious outlet, and the chief examples of them are, the peritoneum, the largest of all, lining the cavity of the abdomen; the two pleura and the pericardium in the chest; and the tunica vaginalis surrounding each of the testicles within the scrotum.

The arachnoid membrane, which is a delicate connective tissue membrane surrounding the brain and spinal marrow in the bony cavities in which they are contained, was formerly reckoned amongst the serous membranes; but neither in the details of its structure, in its general disposition, nor in its development does it correspond with the other serous membranes. It is, therefore, no longer classed with them, but will be described with the other membranes investing the brain and spinal cord.

Form and arrangement.—In all cases a serous membrane has the form of a closed sac, one part of which is applied to the walls of the cavity which it lines, the parietal portion; whilst the other is reflected over the surface of the organ or organs contained in the cavity, and is therefore named the reflected or visceral portion of the membrane. Hence the viscera in such cavities are not contained within the sac of the serous membrane, but are really placed behind or outside of it; seeming to push inwards the part of the membrane which immediately covers them, some organs receiving in this way a complete, and others only a partial and sometimes very scanty investment.

In passing from one part to another, the membrane frequently forms folds which. in general receive the appellation of ligaments, as, for example, the folds of peritoneum passing between the liver and the parietes of the abdomen, but which are sometimes designated by special names, as in the instances of the mesentery, mesocolon, and omentum.

The peritoneum in the female sex, is an exception to the rule that serous mem

branes are perfectly closed sacs, inasmuch as it has two openings by which the Fallopian tubes communicate with its cavity.

A serous membrane sometimes lines a fibrous membrane, as where the serous layer of the pericardium adheres to its outer or fibrous part. Such a combination is often named a fibro-serous membrane.

The inner surface of a serous membrane is free, smooth, and polished; and, as would occur with an empty bladder, the inner surface of one part of the sac is applied to the corresponding surface of some other part; a small quantity of fluid, usually not more than merely moistens the contiguous surfaces, being interposed. The parts situated in a cavity lined by serous membrane, being themselves also covered by it, can thus glide easily against its parietes or upon each other, and their motion is rendered smoother by the lubricating fluid.

The outer surface most commonly adheres to the parts which it lines or covers, the connection being effected by means of areolar tissue, named therefore "subserous," which, when the membrane is detached, gives to its outer and previously adherent surface a flocculent aspect. The degree of firmness of the connection is very various : in some parts the membrane can scarcely be separated; in others its attachment is so lax as to permit easy displacement.

Structure and properties.-Serous membranes are thin and transparent, so that the colour of subjacent parts shines through them. They are tolerably strong,

Fig. 447.-ENDOTHELIAL LAYER OF OMENTUM OF RABBIT. NITRATE OF SILVER PREPARATION.

(E. A. S.)

with a moderate degree of extensibility and elasticity. They are lined on the inner surface by a simple epithelial layer of flattened cells (endothelium, fig. 447), each of which contains a round or oval nucleus with one or two nucleoli, and an intranuclear network. The cells have, according to Klein, a comparatively coarse network of minute fibrils embedded in the otherwise clear cell-substance. The outlines of the cells may readily be brought into view by treatment with nitrate of silver. The lines of junction of the cells which are thus made evident, may be straight and even, but are most commonly slightly jagged or sinuous. Here and there between the cells apertures are to be seen, which are of two kinds. The smaller, which are also the more numerous, are occupied either by an accumulation of the intercellular