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 trabecular or prolongations from the capsule are found the branches of the splenic artery, dividing into smaller twigs without anastomosis. On leaving the trabecular 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.

(a) Trabecule of the Spleen. (b) Artery cut obliquely. {Cadiat).

Fig. 161. (a) Trabecule of the Spleen. (b) Artery cut obliquely. {Cadiat).

Between the trabecular are found two distinct kinds of tissue: (1) Rounded masses of lymphoid tissue, called Malpighian 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 haemoglobin, 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.

Reticulum of the Spleen Pulp injected with colorless gelatine.

Fig. 162. Reticulum of the Spleen Pulp injected with colorless gelatine. (Cadiat).

(a) Meshes made of endothelium.

(6) Lacunar spaces, through which the blood flows. (c) Nuclei of endothelium.

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 haemoglobin. 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, leucocythaemia, 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.