SPLEEN, a vascular abdominal organ which, in man at least, is now generally re garded by physiologists as forming one of the ductless glands, and which is accordingly classed with the thyroid gland, thymus, and suprarenal capsules. All vertebrates — with the exception of the lancelet, and probably the lampreys, lepidosirens (or mudfishes), and the ceratodus or barramunda— possess a spleen, which is absent from the subkingdom in certain aber rant fishes only. In man, the spleen lies in the upper part of the abdomen, and is situated in the left hypochondriac region, contiguous to the cardiac or gullet end of the stomach. Its outer surface is smooth, and lies in contact with the under surface of the diaphragm or midriff, this latter muscle separating the spleen from the 9th, 10th and 11th ribs of the left side. Externally the spleen is covered by the perito neum, and is connected with the stomach by the omentum, known as the gastrosplenic band. Its internal aspect is concave, and is divided by a longitudinal groove or fissure named the hilum. The blood-vessels and nerves of the spleen enter and leave the organ by the hilum. In feriorly the internal surface of the spleen is in contact with the pancreas (q.v.) or sweet bread, and posteriorly with the suprarenal capsule of the left kidney. The upper end is of rounded conformation, and is thick; while the lower extremity is pointed, and is in con tact with the colon (q.v.). A suspensory ligament or special fold of peritoneum attaches the spleen to the under surface of the midriff. The size of the spleen appears to vary much. Its average size in the healthy adult is about five inches in length, by three or four inches broad, and one to one and one-half inches in thickness. Its average weight is seven ounces.
The spleen is invested by an outer serous membrane formed by the peritoneal layers. It covers the entire surface. Below the serous coat is a second investment of fibrous nature. This is of elastic structure, and forms the frame work or supporting fibres of the internal struc ture. The essential spleen matter is called spleen pulp. It is of a dark brownish-red color, and when microscopically viewed is seen to be composed of colored parts, consisting of red blood corpuscles and other cells of colored na ture; while other bodies of deep-red, yellow, or black hue, existing singly or aggregated together, may be seen amid the spleen pulp. The color
less elements seen in the spleen-structure are granular matters, free nuclei of cells, as well as nucleated cells or The colorless ele ments form about two-thirds of the pulp and are of the same nature as the round white cells of lymphatic glands (q.v.). They are also similar to white blood-corpuscles. When the spleen is cut through, as in a vertical section, a number of opaque bodies of small size, masses of round cells, are seen to be scattered throughout its substance. They are the Mal pighian or splenic corpuscles, which have intimate relations with the veins of the spleen, and re semble lymph-corpuscles (see LYMPH) in form. The splenic artery, supplying the organ with blood, is of large calibre, and pursues a re markably tortuous course within the spleen. The veins of the spleen, like the splenic artery, are of large relatiVe size, and they unite to form a large (splenic) vein, which pours its fluid into the portal vein. The nerves of the organ are derived from the right and left semilunar ganglia, and form the right pneumo gastric nerve.
The purpose and functions of the spleen have been the subject of much speculation; but in modern research has shown that in all prob ability the spleen is the seat of the change and elaboration of the red blood-corpuscles, which form such characteristic elements in vertebrate blood. It is thus to be regarded as a blood gland, or kind of lymphatic gland. The spleen may be removed or extirpated, both from man and lower animals, without any apparent bad consequences. This is explicable on the ground that other glands (thyroid, thymus, or even the ordinary lymphatics) may assume the functions of the absent spleen.
Of the diseases to which the spleen is liable inflammation and enlargement are the most common. Inflammation (splenitis) may result in splenic abscess and gangrene; while the organ may be affected by various forms of tubercular and syphilitic disease, and is liable to be ruptured by violence— as from a direct kick or blow. Enlargement (known as ague cake) results from chronic ague or intermittent fever, and may sometimes be met with in pregnant women, and in leucannia (q.v.). See