The spleen is a large blood-vascular organ closely associated with the lymphatic system.

Position

The spleen is situated in the dorsal part of the left cephalic segment of the abdominal cavity so deeply placed against the diaphragm and dorsal to the stomach and colon as to be invisible from the ventral surface of the body when the abdominal cavity is opened. It is mainly in the left hypochondriac region but its deepest and most cephalic part extends also into the epigastric region. It is obliquely placed with its long axis corresponding approximately to the line of the caudal ribs. It tends to become more vertical when the stomach is fully distended but when the stomach is empty and the colon distended it assumes a more horizontal position. Changes in the attitude of the body also cause slight alterations in the situation of the spleen. It moves with the excursions of the diaphragm in expiration and inspiration.

The colour of the spleen is, in life, a dark bluish-red or brownish-red, but after death it becomes darker with a more bluish or violet tint.

The size of the spleen is perhaps more variable than that of any other large organ in the body. Not only does the size differ in different individuals but it changes greatly with the blood content in the same individual. There is a distinct expansion for a time after each meal and the spleen contracts and expands rythmically.

Wedge-shaped Spleen, Viscebal Surface. Diaphragmatic surface Margo anterior

In the adult it usually measures 10 to 15 cm. in length, 7.5 to 10 cm. in breadth, and 2.5 to 4 cm. in thickness. The weight usually ranges from 150 to 225 gm. At birth it represents from jitg to jjo of the total body weight and this porportion is maintained wuthout much variation until the age of fifty years, when (like the lymphoid organs in general) it begins to diminish in size. This diminution continues until in the very old it represents but y J „ of the body weight. There is no great difference in relative size in the two sexes.

Tetrahedral-shaped Spleen, Visceral Surface.

The spleen is somewhat soft and very friable. It is elastic, extensible, contractile, and extremely vascular.

Shape

In form the spleen varies greatly. This is due largely to its softness which permits considerable modifications by the pressure of the distended or contracted surrounding hollow viscera. When in situ with the stomach distended, its shape may be compared to a blunt spherical wedge with a concave apex and rounded extremities, and possessing therefore three surfaces; but when the stomach is contracted and the left flexure of the colon distended an additional surface is produced and its shape becomes tetrahedral. Inter-

Spleen Showing Tubercle on the Intermediate Border.

mediate forms between these extremes are produced by variations in the degree of distention of stomach and colon. The spleen presents two aspects: lateral or parietal, against the diaphragm; and medial or visceral, toward the abdominal cavity. In its usual wedge form the three surfaces of the spleen are diaphragmatic, gastric, and renal. There are three borders, anterior, posterior, and intermediate; and two extremities, superior and inferior.

Cross-section of the Body at the Lower Part of the Epigastric Region. (Rüdinger.)

The diaphragmatic surface [facies diaphragmatica] is a smooth convex surface with an irregularly oval outline, in the wedge-shaped spleens wider cephalically, but in the tetrahedral-shaped spleens wider caudally. It looks dorsally toward the left and somewhat cephahcally.

It lies against the diaphragm over an area opposite the ninth, tenth, and eleventh ribs and the intervening intercostal spaces, with its long axis corresponding in a general way to the course of the ribs. Although it is separated from the ribs by the peritoneum, the diaphragm, and the left pleural cavity (cephalically also by the left lung), the ribs sometimes make impressions upon it.

The gastric surface [facies gastrica] is a semilunar-shaped surface, concave cephalo-caudally and from side to side, wiiich looks ventrally to the right and somewhat caudally. Nearly parallel with the dorsal boundary of this surface is a narrow depression usually formed by a series of pits, as a rule six or eight, which together form the hilus of the spleen [hilus lienis]. In this situation the vessels and nerves enter and leave the spleen, the vein being dorsal.

Sagittal Section through the Left Side of the Body, Showing the Relations of the Spleen.

IX, X, XI, XII, corresponding ribs. 1, Left kidney; 2, spleen; 3, pancreas; 4, splenic vessels; 5, transverse colon; 6, stomach; 7, left lobe of liver; 12, lung; 14, heart; 16, diaphragm. (Testut and Jacob.)

When the stomach is distended it is in contact with the major part of the gastric surface; the left flexure of the colon forming an impression upon a small area near the caudal extremity and the taU of the pancreas, as a rule, resting against a narrow area dorsal to the hilus or just cephalic to the colon. When the stomach is empty and contracted and the colon distended the size of the gastric area is considerably decreased and the relative size of the coUc impression greatly increased so as to form upon the spleen in this situation a colic or basal surface. The stomach is, however, at all times in contact with some part of the spleen.

The renal surface [facies renalis] the smallest of the three surfaces, shorter as well as narrower than the gastric surface, is an oblong, flat or slightly concave area, which faces dorsally, to the right and slightly caudally. It is in relation with the anterior surface of the left kidney.

In some cases the cephalic third of the renal surface is also in relation with the anterior surface of the suprarenal gland. It is separated from these latter structures, however, by the renal adipose capsule as well as by the peritoneum. The tail of the pancreas in some cases is in contact with a small area on the ventral part of this surface. In fat individuals these relations are not as intimate as the relations with other organs because of the large amount of suprarenal fat.

The anterior border [margo anterior] is clearly defined, thin, sharp, and more or less convex. It is marked in over 90 per cent, of the cases by one or more transverse or oblique notches, especially in its cephalic part. It is placed between the diaphragm and the stomach and separates the diaphragmatic from the gastric surface.

The posterior border [margo posterior] is rounded, shorter, and straighter than the anterior border and is notched in less than a third of the cases. It separates the diaphragmatic from the renal surface and is lodged in the angle between the left kidney and the diaphragm.

The intermediate border is a blunt ridge dorsal to the hilus, separating the gastric from the renal surface.

It may be clearly defined or more or less obscure and often shows a marked tubercle. When the stomach is contracted and the colon distended this border divides caudally into ventral and dorsal limbs both of which may be well marked or either may be deficient depending on the direction and degree of pressure of surrounding organs. When well marked there is produced at the point where the two limbs diverge a more or less marked projection, the intermediate extremity or angle.

The superior extremity [extremitas superior], usually larger than the inferior extremity in the wedge-shaped spleens but smaller in the tetrahedral form, is rounded and bent medially. It extends as high as the tenth thoracic vertebra and lies 1 to 2 cm. from the vertebral column.

The inferior extremity [extremitas inferior], also somewhat rounded, is directed toward the left and caudally. It is in relation with the phrenicocolic ligament.

When the stomach is contracted and the colon distended the inferior extremity becomes much broader, in extreme cases forming a distinct inferior border ending ventrally in the anterior margin as an anterior extremity and dorsally in the posterior margin as the posterior extremity.

In the tetrahedral-shaped spleen the additional surface produced by the pressure of the colon is known as the basal or colic surface. This varies in size reciprocally with the degree of pressure of colon and stomach.

When well developed the cohc surface is concave and is separated from the renal and gastric surfaces by the more or less sliarply defined dorsal and ventral limbs of the intermediate border and separated from the diaphragmatic surface by an inferior margin produced from the broadened inferior extremity. The left flexure of the colon is in contact with the greater part of this surface, but the pancreas also usually hes against it in its cephahc part.

Peritoneal relations

The surface of the spleen is completely covered, except for a small area at the hilus, by a peritoneal coat, the tunica serosa. Ventral to the hilus a double layer of peritoneum is prolonged from the spleen to the left side of the greater curvature of the stomach and the left edge of the ventral layer of the great omentum, forming the gastrolienal ligament which contains the short gastric arteries and veins. Dorsally a second double layer of peritoneum extends from the hilus to the ventral surface of the kidney and the caudal surface of the diaphragm forming the phrenicolienal (lienorenal) ligament. This ligament encloses the splenic artery and veins as they pass to and from the spleen. It is also between the two layers of peritoneum of this ligament that the tail of the pancreas reaches the spleen. Except by these two Ligaments the spleen has normally no attachment to the abdominal wall or to any of the surrounding viscera. The gastroHenal, and more especially the phrenicohenal ligament, serve in a measure to anchor the spleen in its place in the abdominal cavity but in addition to these the spleen is supported by a fold of peritoneum which e.xtends from the left cohc flexure to the parietal peritoneum over the diaphragm, the phrenico-colic ligament. This serves as a shng in which the inferior extremity of the spleen rests. The spleen, however, is held in position in the abdominal cavity mainly by the intraabdominal pressure.

Topography

The superior extremity of an average-sized spleen is located between the angle and tubercle of the tenth rib on the left side and about 3 to 4 cm. from the median line on a level with the spinous process of the ninth thoracic vertebra. In the majority of cases, it does not extend more than 2 cm. either cephalic or caudal to a transverse plane at the level of the infra-sternal notch. The inferior extremity reaches nearly to the midaxillary lioe in the tenth intercostal space and 10 to 15 cm. from the superior extremity. The long axis therefore corresponds nearly to the shaft of the tenth rib. The posterior border lies beneath the cephalic border of the eleventh rib. The whole spleen (unless enlarged) lies dorsal to a plane passed through the midaxillary lines and is lateral to a line from the left sternoclavicular joint to the tip of the left eleventh rib. In deep inspiration the spleen is greatly depressed and if enlarged may be felt beneath the ribs.

Variations

From the mean weight between 150 and 200 gm. there are wide variations. It is not rare to find spleens weighing 80 to 100 gin. and they are recorded as light as 10 and 20 gm. On the other hand, spleens weighing 3000 to 4000 gm. are sometimes foimd. These are usually, however, associated with an acute infectious disease, such as malaria or typhoid fever, or a progressive metamorphosis, such as leukemia.

Congenital absence of the spleen is one of the rarest anomaUes. The presence of more than one spleen is the commonest anomaly of the spleen. Adami has found accessory spleens to occur in 11 per cent, of all autopsies. They are round or oblong and vary in size from a pea, or smaller, to a walnut. There are most often one or two but there may be twenty or more. They are found near the hilus on the dorsal side of the gastroUenal ligament, less often, in the great omentum, in the mesentery, on the wall of the intestine, or in the tail of the pancreas.

In certain cases the left lobe of the liver is very long and prolonged far to the left and separates the spleen from the diaphragm. This is the rule in the foetus and is often found in the infant but is exceptional in the adult.

Exceptionally the spleen may be placed far caudal to the normal situation extending into the iliac region and even into the pelvis. This is due in part to congenital laxness of the supports, also to increase in weight. The spleen has been found in almost every part of the abdominal cavity and in transposition of the viscera it is upon the right side.

One or more notches on the anterior border are present according to Parsons in 93 per cent, of the oases, two or more in 66 per cent., but five, six, or seven much more rarely. On the posterior border notches are found in 32 per cent, of the cases, and on the inferior border in 8 per cent. In 20 per cent, of the cases a marked fissure, occasionally more than one, is found on the diaphragmatic surface. Most frequently it begins at one of the notches in the posterior border and passes for a distance across the surface, rarely reaching the anterior border. Occasionally such a fissure starts from the anterior border and rarely there is such a fissure connecting with neither border.

Portion op Section of the Spleen of an Adult Man. (Lewis and Stohr.)

Structure

The peritoneal covering of tlie spleen, tunica serosa, is intimately bound to the underlying, whitish, highly elastic fibrous capsule, the tunica albuginea. This is composed mainly of white fibrous connective tissue but contains numerous fine elastic fibers, and a few smooth muscle fibers. It is much thicker than the serous covering and completely invests the spleen. From its dee]) s\irt':ice the tunica albuginea gives off into the interior numerous trabecule, trabeculae lienis, which join with one another and form a framework in which course the blood-vessels, more especially the veins. It is through the contraction of the smooth muscle fibers in the tunica albuginea and trabeculae, that the regular periodic contraction and expansion of the spleen is produced.

In the meshes of the trabecular network, lymphoid tissue which forms the proper splenic tissue, the pulpa lienis, is located. This is soft, friable, and dark brownish or bluish-red in colour. In this, in a fresh spleen, are seen small round whitish or greyish masses from .25 to 1.5 mm. in diameter, the Malpighian corpuscles [noduli lymphatici lienales; Malpighii].

The trabecula3 are in connection with a reticular network which permeates the spleen substance or spleen-pulp. Mall has shown that the trabeculse and vascular system together outline masses of spleen-pulp about 1 mm. in diameter, known as splenic lobules. Each lobule is bounded by three main trabeculae, from each of which secondary trabeculoB pass into the substance of the lobule incompletely subdividing it into compartments, filled with splenic pulp, arranged in the form of anastomosing columns or cords and designated as pulp-cords. The branches of the splenic artery, after coursing for a short distance in the main trabeculae, leave these, and, after further division, become surrounded with a layer of adenoid tissue, which layer presents here and there irregular thickenings forming the Malpighian corpuscles. An arterial branch, surrounded with adenoid tissue, enters the apex of a splenic lobule, constituting its intralobular vessel, which, soon after entering the lobule, loses its adenoid sheath and then sends a branch to each of the above-mentioned compartments. These branches do not anastomose. They give off terminal branches which course in the pulp-cords, form dilations, ampuUse, and terminate directly or indirectly in the large venous spaces found between the pulpcords. From the latter the blood passes, by means of small intralobular veins, to interlobular veins situated in the trabeculae bounding the lobules. Some of the ampullae are connected with one another by capillary branches.

Blood-supply

The spleen receives its blood from the splenic artery, which is very large in proportion to the size of the organ it supplies. It divides in the phrenicolienal ligament into from three to six or eight branches, rami lienales, which enter the spleen at the hilus. After entering the spleen the arteries divide and subdivide and run to their termination in the ampullae without anastomosing. They form what are known as terminal arteries. The main splenic artery is very tortuous. The vein, vena lienalis, leaves the spleen usually by the same number of branches as the entering artery. These imite in the phrenicohenal ligament to form a large trunk which is straighter than the splenic artery and hes caudal to it.

The lymphatics

A superficial and a deep set of lymphatics have been described in the spleen. The former is said to form a plexus beneath the peritoneum and the latter to be derived from the fine perivascular spaces in the adenoid tissue around the vessels. From these several trunks arise and joining at the hilus pass between the layers of the phrenicolienal ligament to empty into the lymph-glands dorsal to and around the cephalic border of the tail of the pancreas. The presence of both superficial and deep sets of lymphatics in the human spleen has been denied by some investigators. According to Mall, there is no deep set.

The nerves

The nerves are derived from the right vagus and from the coeliac plexus. They enter the spleen at the hilus, accompanying the branches of the lienal artery. They are composed mostly of non-medullated fibers which form a rich plexus around the arteries supplying the muscular fibers in the media while a second group has been traced to the muscular fibers of the trabecule.

Development of the spleen

The first anlage of the spleen is seen in the fifth week of foetal life as a swelling on the dorsal (left) surface of the mesogastrium. This is due to an increase in the mesenchymal cells as well as to a thickening of the coelomic epithelium. This latter becomes stratified, and indistinctly differentiated from the underlying embryonic connective tissue through the transformation of the deepest of the epithelial cells into mesenchymal cells. As development proceeds the thickened mass becomes entirely isolated and the ccelomic epithelium covers it as a single layer.

The arteries are seen first as a capillary network throughout the organ which considerably later become arranged as tufts of widened capillaries, the anlage, of the vascular structural unit. These spherical groups of arterial capillaries leading by wide openings into a wide meshed venous plexus are boimded by trabecule from the capsule. The number of structural units in the spleen seems to be fixed fairly early but the size and complexity changes greatly. The spherical mass with a single central artery changes to the adult condition where the central artery gives off side branches, each of which has a spherical mass of capillaries, and the pulp intervenes between the artery and the vein so that the capillary circulation of the early embryo becomes the cavernous circulation of the adult. The lienal lymphatic nodules of Malpighi and the splenic pulp appear only in the latter half of embryonic life.