adventitious coat round their small branches, and even on some of the

capillaries (page clxxix).

Fig. CIV. SECTION OF MEDULLARY SUBSTANCE OF AN INGUINAL GLAND OF THE OX (magnified 90 diameters.)

a, a, glandular substance or pulp forming rounded cords joining in a continuous net (dark in the figure); c, c, trabecule; the space, b, b, between these and the glandular substance is the lymph-sinus, washed clear of corpuscles and traversed by filaments of retiform connective tissue (after Kölliker).

Fig. CV.-A VERY SMALL PORTION OF THE MEDULLARY SUBSTANCE FROM A MESENTERIC GLAND OF THE OX (magnified 300 diameters).

d, d, trabecule; a, part of a cord of glandular substance from which all but a few of the lymph corpuscles have been washed out to show its supporting meshwork of retiform tissue and its capillary blood vessels (which have been injected, and are dark in the figure); b, b, lymph-sinus, of which the retiform tissue is represented only at c, c (after Kölliker).

As to the lymphatics of the gland, it seems now to be tolerably well made out, that the afferent vessels, after branching out upon and in the tissue of the capsule, send their finer branches through it to open into the lymph-sinuses of the cortical alveoli, and that the efferent lymphatics begin by fine branches leading from the lymph-sinuses of the medullary part, and forming at the hilus a dense plexus of tortuous and varicose-looking vessels, from which branches proceed to join the larger efferent trunks. The lymphsinus, therefore, forms a channel 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 epithelium, and the sinus is lined throughout its whole extent with a similar epithelium, consisting, as in the commencing lymphlacunse, of a single layer of flattened cells.

It is not unreasonable to presume that in the proper glandular substance, there is a continual production of lymph-corpuscles, most probably by fissiparous multiplication, which pass into the lymph sinus, and that fresh corpuscles are thus added to the lymph as it passes through a gland; and this view is supported by the fact, that the corpuscles are found to be more abundant in the lymph or chyle after it has passed through the glands (see page 1.). It has been alleged, moreover, that the lymph, after passing the glands, is richer in fibrin, and therefore coagulates more firmly. In any case it is plain that the numerous blood capillaries distributed in a gland, must bring the blood into near relation with the elements of the lymph, and the latter fluid, as it must move very slowly through the relatively wide space within the gland, is thus placed in a most favourable condition for some not improbable interchange of material with the blood.

Termination.-The absorbent system discharges its contents into the veins at two points, namely, at the junction of the subclavian and internal jugular veins of the left side by the thoracic duct, and in the corresponding part of the veins of the right side by the right lymphatic trunk. The openings, as already remarked, are guarded by valves. It sometimes happens that the thoracic duct divides, near its termination, into two or three short branches, which open separately, but near each other; more rarely, a branch opens into the vena azygos—indeed the main vessel has been seen terminating in that vein. Again, it is not uncommon for larger branches, which usually join the thoracic duct, to open independently in the vicinity of the main termination; and this is more apt to happen with the branches which usually unite to form the right lymphatic trunk. By such variations the terminations in the great veins are multiplied, but still they are confined in man to the region of the neck; in birds, reptiles, and fish, on the other hand, communications take place between the lymphatics of the pelvis, posterior extremities and tail, and the sciatic or other considerable veins of the abdomen or pelvis.

The alleged terminations of lymphatics in various veins of the abdomen, described by Lippi as occurring in man and mammalia, have not been met with by those who have since been most engaged in the prosecution of this department of anatomical research, and accordingly his observations have generally been either rejected as erroneous, or held to refer to deviations from the normal condition.* But, while such (extra-glandular) terminations in other veins than those of the neck have not been generally admitted, several anatomists of much authority have maintained that the lacteals and lymphatics open naturally into veins within the lymphatic glands. This latter opinion which has been strenuously advocated by Fohmann in particular, is based on a fact well known to every one conversant with the injection of the vessels in question, namely, that the quicksilver usually employed for that purpose, when it has entered a gland by the inferent lymphatics, is apt to pass into branches of veins within the gland, and thus finds its way into the large venous trunks in the neighbourhood, in place of issuing by the efferent lymphatic vessels. But, although it, of course, cannot be doubted that, in such cases, the mercury gets from the lymphatics into the veins, no one has yet been able to perceive the precise mode in which the transmission takes place, and, looking to the circumstances in which it chiefly occurs, it seems to be more probably owing to rupture of contiguous lymphatics and veins within the glands, than to a natural communication between the two classes of vessels in that situation.

Lymphatic hearts.-Müller and Panizza, nearly about the same time, but independently of each other, discovered that the lymphatic system of reptiles is furnished, at its principal terminations in the venous system, with pulsatile muscular sacs,

* In a communication inserted in Müller's Archiv. for 1848, p. 173, Dr. Nuhn, of Heidelberg, affirms the regular existence of these abdominal terminations, and refers to three instances which he met with himself. In two of these, the lymphatics opened into the renal veins, and in the other into the vena cava.

which serve to discharge the lymph into the veins. These organs, which are named lymph-hearts, have now been found in all the different orders of reptiles. In frogs and toads two pairs have been discovered, a posterior pair, situated in the sciatic region, which pour their lymph into a branch of the sciatic or of some other neighbouring vein, and an anterior more deeply-seated pair, placed over the transverse process of the third vertebra, and opening into a branch of the jugular vein. The parietes of these sacs are thin and transparent, but contain muscular fibres of the striated kind, freely ramifying, decussating in different layers, as in the blood-heart. In their pulsations they are quite independent of the latter organ, and are not even synchronous with each other. In salamanders, lizards, serpents, tortoises, and turtles, only a posterior pair have been discovered, which, however, agree in all essential points with those of the frog. In the goose, and in other species of birds belonging to different orders, Panizza discovered a pair of lymph-sacs opening into the sacral veins, and Stannius has since found that these sacs have striated muscular fibres in their parietes; but, although this observer, in some cases, exposed them in the living bird, he was not able to discover any pulsation or spontaneous movement in them. Nerve-fibres, both dark bordered and pale, have been observed in the lymph-hearts of the frog, and also nerve-cells in those of the common tortoise. (Waldeyer.)

Development of lymphatic vessels.-Kölliker has observed the formation of lymphatics from ramified cells in the tails of young salamander·larvæ. He states that the process takes place nearly in the same manner as in the case of sanguiferous capillaries; the only notable difference being, that whilst the growing lymphatics join the ramified cells, and thus extend themselves, their branches very rarely anastomose or become connected by communicating arches. The soundness of his conclusions has, however, been called in question and the subject requires further elucidation. New-formed lymphatics have been injected in adhesions between inflamed serous membranes.

SEROUS MEMBRANES.

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

Form and arrangement.—In all these cases the 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; merely pushing inwards, as it were, the part of the membrane which immediately covers them, some organs receiving in this way a complete, and others but 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, meso-colon, and omentum.

The peritoneum, in the female sex, is an exception to the rule that serous membranes 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 arachnoid lines the dura mater, or where the serous layer of the peri

cardium 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 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 scarce be separated; in others, its attachment is so lax as to permit of easy displacement. The latter is the case in the neighbourhood of the openings through which abdominal herniæ pass, and accordingly when such protrusions of the viscera happen to take place, they usually push the peritonæum before them in form of a hernial sac.

The visceral portion of the arachnoid membrane is in some measure an exception to the rule of the outer surface being everywhere adherent; for in the greater part of its extent, it is thrown loosely round the parts which it covers, a few fine fibrous bands being the sole bond of connection; and a quantity of pellucid fluid is interposed, especially in the vertebral canal and base of the cranium, between the arachnoid and the pia mater, which is the membrane immediately investing the brain and spinal cord.

Structure and properties.—Serous membranes are thin and transparent, so that the colour of subjacent parts shines through them. They are tolerably strong, with a moderate degree of extensibility and elasticity. They consist of, 1st, a simple layer of scaly epithelium already described and figured (fig. xx.), which, however, is in part ciliated on the serous membrane lining the ventricles of the brain and central canal of the spinal cord. 2ndly, the fibrous layer. This consists of fine but dense areolar connective tissue, which is, as usual, made up of bundles of white filaments mixed with fine elastic fibres; the former, when there are two or more strata, take a different direction in the different planes; the latter unite into a network, and, in many serous membranes, as remarked by Henle, are principally collected into a reticular layer at the surface, immediately beneath the epithelium. The constituent connective tissue of the serous membrane is of course continuous with the usually more lax subserous areolar tissue connecting the membrane to the subjacent parts. Where the arachnoid membrane lines the dura mater, and possibly also in some other cases, the fibrous layer usually belonging to the serous membrane is wanting, its place being supplied by the fibrous membrane beneath, on which the epithelium is immediately applied.

Blood-vessels ending in a capillary network with comparatively wide meshes pervade the subserous tissue and the tissue of the serous membrane. Plexuses of lymphatics also exist in the subserous tissue, but not under every part of the membrane; in the costal pleura, for example, the lymphatics are confined to the parts which cover the intercostal and sterno-costal muscles. When present the lymphatics extend in form of fine superficial plexuses rough the fibrous layer of the membrane to its surface, immediately neath the epithelium (Dybkowski), and may then open into the serous

SYNOVIAL MEMBRANES.

cxciii

cavity by cognizable apertures, as already stated. Fine nervous fibres, with nerve-cells in some places, have been described by several anatomists, in or immediately beneath the serous membranes of various regions; nevertheless it would seem, that when in a healthy condition these membranes possess little or no sensibility; they are altogether devoid of vital contractility. Fluid.-The internal surface of serous cavities is moistened and lubricated with a transparent and nearly colourless fluid, which in health exists only in a very small quantity. This fluid, which is doubtless derived from the blood-vessels of the membrane, has been commonly represented as similar in constitution to the serum of the blood. But it was long since remarked by Hewson (and a similar opinion seems to have been held by Haller and Monro), that the fluid obtained from the serous cavities of recently-killed animals coagulates spontaneously, and thus resembles the lymph of the lymphatic vessels, and, we may add, the liquor sanguinis or plasma of the blood, the coagulation being, of course, due to the presence of fibrin, or of its two constituents fibrinogen and globulin. Hewson, who regarded the fluid as lymph, found that the coagulability diminished as the quantity increased. In confirmation of Hewson's statement, I may mention that I have always found the fluid obtained from the peritoneal cavity of rabbits to coagulate spontaneously in a greater or less degree. Hewson made his observations on the fluid of the peritonæum, pleura, and pericardium, in various animals, viz., bullocks, dogs, geese, and rabbits.*

When the fluid gathers in unusual quantity as in dropsies, it rarely coagulates spontaneously on being let out; but will often yield a coagulum on the addition of globulin as already stated (page xxxviii.). From this it may be inferred that fibrinogen is present, but not the globulin (fibrino-plastin) requisite to generate fibrin.

The identity in character of the fluid of serous cavities and the lymph plasma is, it need scarcely be remarked, in keeping with the notion of their being great lymphspaces in open connection with lymphatic vessels. But this view is quite reconcilable with the mechanical purpose commonly ascribed to these membranes, of lubricating and facilitating the movement of mutually opposed surfaces.

When a serous membrane is inflamed, it has a great tendency to throw out coagulable lymph (or fibrin) and serum, the two constituents of the blood-plasma, the former chiefly adhering to the inner surface of the membrane, whilst the latter gathers in its cavity. The coagulable lymph spread over the surface, in form of a "false membrane," as it is called, or agglutinating the opposed surfaces of the serous sac and causing adhesion, becomes pervaded by blood-vessels, and in process of time converted into areolar tissue.

Breaches of continuity in these membranes are readily repaired, and the new-formed portion acquires all the characters of the original tissue.

SYNOVIAL MEMBRANES.

Resembling serous membranes in general form and structure, the synovial membranes are distinguished by the nature of the secretion which lubricates their surface, for this is a viscid glairy fluid resembling the white of an egg, and thence named synovia.

These membranes line the cavities of joints, and are interposed between moving parts in certain other situations; being in all cases intended to lessen friction, and thereby facilitate motion. They are composed of a scaly epithelium, which may consist of several strata, and a layer of dense areolar tissue pervaded by vessels and attached by tissue of the same kind to the parts beneath.

The different synovial membranes of the body are referred to three classes, viz., articular, vesicular, and vaginal.

1. Articular synovial membranes, or Synovial capsules of joints.

These

* See Hewson's Works, published by the Sydenham Society, p. 157, with some important remarks in notes xviii. and lxviii., by the editor, Mr. Gulliver.