Spleen. This organ, present in vertebrates from the fishes upwards, has no doubt very important functions to perform, but about these we know as yet very little indeed. It is of the size and shape of a rather long halfpenny bun, and is situated in the left hypochondriac region, clasping by its flat surface the cardiac portion of the stomach (see figure at article ABDOMEN). Unlike the liver and pancreas, it has no duct and manufactures no juice, being connected with the rest of the body by its blood-vessels, nerves, and lymphatics: these enter the organ at its hylus and ramify within it. The spleen is invested by a capsule consisting chiefly of muscular tissue, and from this capsule muscular processes called trabeculæ run into the interior of the organ supporting its delicate pulp. As one would expect from its structure, the organ can expand and contract, and this it does from a variety of causes—for instance, after a meal the organ expands, reaching its maximum in about five hours, then contracting again. In diseased conditions it may expand to several times its normal size, which would not be possible were its capsule of fibrous tissue like those of other glands. The blood-vessels enter the spleen, and the arteries become invested by curious masses of tissue called adenoid, and these little masses, the splenic corpuscles, are of about the size of millet seed, and quite visible to the naked eye on cutting open the organ.
If the spleen of a man or an ox be cut open it will appear of a soft pulpy consistence and deep crimson in colour, with little white patches, these splenic corpuscles, scattered through it. On putting the spleen under a running jet of water the soft spleen pulp infiltrated with blood will be washed away, leaving behind the tough capsule, the branching trabeculæ, and the blood-vessels with some of these splenic corpuscles attached to them. The blood-vessels end in the tissue of the spleen, and those that terminate within the splenic bodies do so in the usual way, passing into true capillaries. Within the spleen pulp, however, which forms the chief part of the organ, the arteries open directly into the loose tissue forming the pulp, so that the blood percolates through this before leaving the organ by the veins. The blood thus becomes intimately associated with this pulpy tissue, and becomes modified by it, as we shall see. It is highly probable that the chief use of the spleen is to modify the blood passing through it, and hence it is spoken of as a blood-gland, in contradistinction to a digestive gland, which pours its secretion into the digestive tract, and aids the processes which go on there. It is very probable that the spleen has the power of arresting and destroying the old worn-out red blood-corpuscles as they pass through it, for within the spleen itself evidence of their destruction is found in the large quantity ever present of iron and other bodies, which would result from their dissolution. Moreover there is strong reason for supposing that the active agents in this destruction are the cells present in the spleen pulp, some of which are generally found with bits of the red blood-corpuscles, and pigmented masses derived from them, within their bodies. But the spleen not only destroys blood-corpuscles; it forms new ones, and these are poured out of the pulp into the splenic vein, and are carried off in the general circulation. Most of these new corpuscles appear to be of the white variety, for an abnormally large number of these are by some observers found in the splenic vein, these white corpuscles changing, aided by the spleen itself, into red ones.
There is little doubt that in a general way the spleen is a blood-modifying gland, and in diseases such as splenic leucocythemia it becomes enormously enlarged, and produces large numbers of white corpuscles. In intermittent fever it is also enormously enlarged, forming the ague-cake. It may, however, be excised, even in the case of man, without producing fatal consequences, the lymphatic glands enlarging and the bone marrow undergoing changes, probably to enable them to compensate for the loss of the other organ.
The spleen was long supposed to be the seat of some of the less amiable emotions—envy, malice, &c.