developed in the foetus, and increases in size for a couple of years after birth; but it gradually diminishes in bulk and loses its original structure during the later periods of childhood, so as to become completely degenerated and fatty in the adult. composed of numerous little follicles of lymphoid tissue collected into groups or lobules connected to a kind of central stalk. The lymphoid follicles of the young thymus have some likeness to those of the intestinal tract, but they differ from these agminate glands not only in arrangement but also in having peculiar small nests of large cells (corpuscles of Hassall) in the midst of the adenoid tissue of which they are made up. On account of the structure of the lobules being so nearly identical with that of a lymphatic gland, and from its great richness in lymphatic vessels, the thymus is said to be related to the lymphatic system, and is supposed to play an importent part in the elaboration of the blood during the earlier stages of animal life.

SPLEEN.

Structure. The spleen also resembles a lymphatic organ in structure, but differs from it in the relation borne by the blood

to the elements of the follicular tissue. It is encased in a strong capsule made of fibrous tissue and unstriated muscle cells. From this many branching prolongations pass into the substance of the

organ, so as to traverse the soft, red, spleen pulp. In these trabeculæ or prolongations from the capsule are found the branches

FIG. 162.

Reticulum of the Spleen Pulp injected with colorless gelatine. (Cadiat.)

(a) Meshes made of endothelium.

of the splenic artery, dividing into smaller twigs without anastomosis. On leaving the trabeculæ the arteries break up suddenly into a brush-like series of small branches, ending in capillaries, which are lost in the pulp where the small veins may be

seen to commence.

Between the trabeculæ are found two distinct kinds of tissue: (1) Rounded masses of lym

(6) Lacunar spaces, through which the phoid tissue, called Malpighian

blood flows.

(c) Nuclei of endothelium.

bodies, scattered here and there through the organ; and (2) the

peculiar soft splenic pulp making up its bulk.

The small rounded masses of lymph follicular tissue are situated on the course of the fine arterial twigs. The delicate adenoid reticulum which holds the lymph cells together is intimately connected with the vessel wall. The pale appearance of these follicles, which distinguishes them from the surrounding splenic pulp, depends on the number of the white cells which are packed in the meshes of this perivascular adenoid tissue.

The splenic pulp consists of a system of communicating lacunar spaces lined with endothelium. Into these spaces the blood is poured from the arteries, and thus mingles with vast numbers of white cells. Besides the ordinary blood discs and the white corpuscles or lymph cells, many peculiar cells are found in the spleen pulp. Some of these look like lymph cells containing little masses of hæmoglobin, and appear to be transitions from the colorless to the red corpuscles, while some small, misshapen, red corpuscles are regarded as steps in a retrograde change in the discs. But few, if any, lymph channels lead from the spleen pulp, and only a relatively small number pass out from the hilus,

so that the splenic artery and vein must be regarded as taking the places of the afferent and efferent lymph channels.

Chemical Composition of the Spleen Pulp.—Chemical examination shows the splenic pulp to have remarkable peculiarities. Although so full of blood, which is generally alkaline, the spleen is acid in reaction, and contains a great quantity of the oxidation products (so-called extractives) commonly found as the result of active tissue change. The chief of these are uric acid, leucin, xanthin, hypoxanthin, inosit, lactic, formic, succinic, acetic and butyric acids. It also contains numerous pigments, rich in carbon, but little known, which are probably the outcome of destroyed hæmoglobin. A peculiarly suggestive constituent is an albuminous body containing iron. The ash is found to contain a considerable quantity of oxide of iron, to be rich in phosphates and soda, with but small quantities of chlorides and potassium.

Changes in the Blood in the Spleen.-If the blood flowing in the artery to the spleen be compared with that in the vein, the difference gives us the changes the blood has undergone in the organ, and hence is of great importance. In the blood of the . vein is found an enormous increase in the number of white corpuscles (1 white to 70 red in the vein, as against 1 to 2000 in the splenic artery). The red corpuscles from the vein are smaller, brighter, less flattened than those of ordinary blood; they do not form rouleaux, and are more capable of resisting the injurious influence of water. The blood of the splenic vein is also said to have a greater proportion of water, and to contain an unusual quantity of uric acid and other products of tissue waste. The amount of blood in the spleen varies greatly at different times. Shortly after meals the organ becomes turgid, and remains enlarged during the later periods of digestion.

Pathological Changes.-The size of the spleen, which may be taken as a measure of its blood contents, is also altered by many abnormal conditions of the blood. Thus, in all kinds of fever, particularly ague and typhoid, and in syphilis, the spleen becomes turgid, and in some of these diseases it remains swollen for some time. In a remarkable disease, leucocythæmia, in which the white blood cells are greatly increased in number, and the red

ones are comparatively diminished, the spleen, in company with the lymphatic glands, is often found to be profoundly altered and diseased, and commonly immensely enlarged; but, on the other hand, advanced amyloid degeneration of the spleen may occur without any notable alteration taking place in the number or properties of the blood corpuscles.

Extirpation of the Spleen.-The spleen may be removed from the body without any marked changes taking place in the blood or the economy generally. It is said that if an animal whose spleen is extirpated be allowed to live for a certain time, the lymphatic glands increase in size, or become swollen.

In attempting to assign a definite function to the spleen all the foregoing facts must be carefully reviewed, and the peculiarity of its (1) structure, (2) chemical composition, (3) the changes the blood undergoes while flowing through it, (4) the variations in blood supply which follow normal and pathological changes in the economy, and (5) the absence of effect following its extirpation, must all be borne in mind.

Its structure teaches us that it is intimately related to lymphatic glands. The Malpighian bodies are simply lymph follicles, and the pulp may be regarded as a sinus like that of a lymph gland, with this difference, that it is traversed by blood instead of lymph. The cell elements found in it indicate that not only white cells are rapidly generated, but also that these cells have some peculiar relationship to hæmoglobin, as they are often found to contain

The varieties in size, form, and general appearance of the red corpuscles can be accounted for by either their destruction or their formation occurring in this organ.

Its chemical composition also shows that certain special changes go on in the pulp, and that probably stages of the construction or destruction of hæmoglobin are here accomplished may be inferred from the peculiar association of iron with albuminous bodies.

From the characters of the blood flowing from the spleen it has been argued that, besides an enormous production of white corpuscles, the destruction of the red discs goes on, while some new discs are formed, probably by means of the white cells

making hæmoglobin in their protoplasm, which, gradually disappearing, leaves only the red mass of hæmoglobin.

The increased activity of the spleen after meals, and in certain abnormal states of the blood, as shown by its containing more blood, distinctly points out that some form of blood elaboration goes on in it, which is nearly related to, or associated with, nutrition.

Section of Spleen through a lymph follicle (Malpighian body) (a) injected to show the vessel (c) entering the follicle, the lymphoid tissue of which is pale in comparison with the pulp (6), the meshes of which are filled with injection. (Cadiat.)

The swelling of the lymphatic glands after extirpation of the spleen confirms its relation to these organs, and the fact is undisputed that it is a source of the white corpuscles of the blood; but the paucity of evidence after this operation as to changes in the number or character of the red discs proves that if the spleen be either the place of origin or destruction of the red corpuscles it cannot be the only organ in which they are produced or destroyed.

GLYCOGENIC FUNCTION OF THE LIVER.

Of all the organs that modify the composition of the blood flowing through them, the liver plays the most important part in elaborating the circulating fluid. The elimination of the various