The Spleen

The spleen (Lien) is the largest of the ductless glands. It is situated chiefly in the left parachondrium, in close relation to the left part of the great curvature of the stomach, to which its long axis corresponds. Its size and weight vary greatly in different subjects, and also in the same subject under different conditions, depending chiefly on the great variability of the amount of blood contained in it. The average weight is about 35 ounces (ca. 1 kg.), its length about 20 inches (ca. 50 cm.), and its greatest width about 8 to 10 inches (ca. 20 to 25 cm.). It is usually bluish-red or somewhat purple in color. In the natural state it is soft and yielding, but not friable.

The weight appears to vary ordinarily from about one to eight pounds, although in large horses the latter figure even may be exceeded without any apparent evidence of disease. There does not seem to be any constant relation to the body-weight. For example, the spleen of a colt about ten months old weighed three and a half pounds, while it often weighs less than two pounds in horses weighing 1000 to 1200 pounds. The chief variation in outline consists of increase of width, especially of the dorsal part.

It extends obliquely in a curved direction from the left crus of the diaphragm and the saccus cæcus of the stomach to the ventral third of the ninth or tenth rib. It presents for description two surfaces, two borders, and two extremities.

The parietal or external surface (Facies parietalis) is convex, and lies chiefly against the diaphragm, but is in direct contact with the upper parts of the last two ribs and to a small extent with the flank at the lumbo-costal angle.

The visceral or internal surface (Facies visceralis) is in general concave. It is divided into two unequal portions by a longitudinal ridge; on this is a groove, the hilus, in which the vessels and nerves are situated. The area in front of the ridge (Facies gastrica) is moulded on the great curvature of the stomach; it is about two inches (ca. 5 cm.) wide. The area behind the ridge (Facies intestinalis) is much more extensive; it is related chiefly to the small colon, the small intestine, and the great omentum. It may be marked by one or two fissures.

The anterior border (Margo anterior) is concave and thin.

The posterior border (Margo posterior) is convex and thin.

The base or dorsal extremity is beveled, and fits into the interval between the left kidney and the left crus of the diaphragm and the psoas major. When hardened in situ, it shows an impression (Facies renalis) where it lies against the kidney. The left extremity of the pancreas touches it also. The anterior basal angle fits in between the saccus cæcus of the stomach and the left kidney; the posterior basal angle usually lies against the flank just behind the last rib.

The apex or ventral extremity is small; it lies—when the stomach is not full—between the left lobe of the liver and the left dorsal portion of the colon; when the stomach is full, the spleen is pushed further back and loses contact with the liver.

The spleen is attached by two peritoneal folds, the suspensory ligament and the gastro-splenic omentum. The suspensory ligament (Lig. suspensorium lienis) attaches the base to the left crus of the diaphragm and the left kidney; it contains a quantity of elastic tissue. The part which passes to the diaphragm is the ligamentum phrenico-lienale, and blends with the gastro-phrenic ligament; the part which goes to the kidney is termed the ligamentum renolienale. The gastro-splenic omentum (Lig. gastrolienale) passes from the hilus to the left part of the great curvature of the stomach. It is narrow above, where it joins the suspensory ligament; below it becomes much wider and is continuous with the great omentum.

Small globular or lenticular masses of splenic tissue may be found in the gastro-splenic omentum. They are termed accessory spleens (Lienes accessoriæ).

Structure.—The spleen has an almost complete serous coat. Subjacent to this and intimately united with it is a capsule of fibrous tissue (Tunica albuginea), which contains many elastic fibers and some unstriped muscular tissue. Numerous trabeculæ (Trabeculæ lienis) are given off from the deep face of the capsule and ramify in the substance of the organ to form a supporting network. In the interstices of this framework is the spleen pulp (Pulpa lienis), a dark red, soft, grumous material. This is supported by a delicate adenoid reticulum, and contains numerous leukocytes, the large splenic cells, red blood-corpuscles, and pigment. The pulp is richly supplied with blood. The branches of the splenic artery enter at the hilus and pass along the trabeculæ. The arteries which enter the pulp have a sheath of lymphoid tissue, which collects on the vessel wall at certain points, forming small lymph nodules, the so-called Malpighian corpuscles (Noduli lymphatici lienales). These are visible to the naked eye as white spots, about as large as the head of a pin. The blood passes into cavernous spaces lined by endothelium which is continuous with the cells of the reticulum of the pulp. From these the veins arise. The splenic vein runs in the hilus in company with the artery and nerves, and joins the posterior gastric vein to form a large radicle of the portal vein.

Blood-supply.—Splenic artery.

Nerve-supply.—Splenic plexus.