The human _spleen_ (Gr. [Greek: splen]) is an oval, flattened gland, of
a dull purple colour, and about 5 in. long by 3 broad, situated in the
upper and back part of the left side of the abdominal cavity. If the
right hand is passed round the left side of its owner's body, as far as
it will reach, it approximately covers the spleen. The long axis of the
organ is obliquely placed so that the upper pole is much nearer the
vertebral column than the lower pole. For practical purposes the long
axis of the left tenth rib corresponds with that of the spleen. There is
an external or parietal surface and an internal or visceral, the latter
of which is again subdivided; these surfaces are limited by ventral and
dorsal borders. The external, parietal, or phrenic surface is convex to
adapt it to the concavity of the diaphragm, against the posterior part
of which it lies; external to the diaphragm is the pleural cavity, and
more externally still, the ninth, tenth and eleventh ribs. The internal
or visceral surface is divided by a prominent ridge into a gastric or
anterior and a renal or posterior surface. Sometimes a triangular
impression called the basal surface is formed at the lower part of the
visceral surface by the left end of the transverse colon, though at
other times no such impression is seen. It is probable that the exact
shape of the spleen depends a good deal on the amount of distension of
the surrounding hollow viscera at the time of death. (For details of the
basal surface see D.J. Cunningham, _Journ. Anat. and Phys._ vol. xxix.
p. 501.) The gastric surface is concave and adapts itself to the fundus
of the stomach, while just in front of the ridge separating the gastric
and renal surfaces is the hilum, where the vessels enter and leave the
organ; in front of this the tail of the pancreas usually touches the
spleen. The renal surface is as a rule smaller than the gastric and,
like it, is concave; it is moulded on to the upper part of the outer
border of the left kidney and just reaches the left adrenal body. The
anterior or ventral border of the spleen has usually two or more notches
in it, though these are often also seen on the dorsal border. The whole
spleen is surrounded by peritoneum, which is reflected off on to the
stomach as the gastro-splenic omentum, and on to the kidney as the
lieno-renal ligament; occasionally the lesser sac reaches it near its
connexion with the pancreas. Small accessory spleens are fairly often
found in the neighbourhood of the spleen, though it is possible that
some of these may be haemo-lymph glands (see LYMPHATIC SYSTEM).
Microscopically the spleen has a fibro-elastic coat in which
involuntary muscle is found (fig. 2). This coat sends multitudes of
fine trabeculae into the interior of the organ, which subdivide it
into numbers of minute compartments, in which the red, highly
vascular, spleen pulp is contained. This pulp contains small spherical
masses of adenoid tissue, forming the Malpighian corpuscles, situated
on the terminal branches of the splenic blood-vessels, together with
numerous cells, some of which are red blood corpuscles, others lymph
corpuscles, others contain pigment granules or fat, while others have
in their interior numerous blood corpuscles. The arteries of the
spleen in part end in capillaries from which the veins arise, but more
frequently they open into lacunae or blood spaces, which give origin
to the veins.
_Embryology._--The spleen is developed in the dorsal mesogastrium (see
COELOM AND SEROUS MEMBRANES) from the mesenchyme, or that portion of
the mesoderm, the cells of which lie scattered in a matrix. Large
lymphoid cells are early seen among those of the mesenchyme, but
whether these migrate from the coelomic epithelium, or are originally
mesenchymal is doubtful, though the former seems more probable. The
network of the spleen seems certainly to be derived from cells of the
mesenchyme which lose their nuclei.
_Comparative Anatomy._--The spleen is regarded as the remains of a
mass of lymphoid tissue which, in a generalized type of vertebrate,
stretched all along the alimentary canal. It is absent as a distinct
gland in the Acrania and Cyclostomata. In the fishes it is closely
applied to the U-shaped stomach, and in some of the Elasmobranchs,
e.g. the basking and porbeagle sharks (Selache and Lamna), it is
divided into small lobules. In Protopterus among the Dipnoi it is
enclosed within the walls of the stomach. In the Anura (frogs and
toads) among the Amphibia it is a spherical mass close to the rectum,
and this may be explained by regarding it as derived from a different
part of the original mass, already mentioned, to that which persists
in other vertebrates. In the Iguana among the reptiles the organ has
many notches, and each one corresponds to the point of entrance of a
vessel. In Mammals the notches, when they are present, so frequently
correspond to the points of entrance of arteries at the hilum that the
present writer believes that the former are determined by the latter
in many cases (see F.G. Parsons on the Notches of the Spleen, _J.
Anat. and Phys._ vol. 35, p. 416; also Charnock Bradley, _Proceedings
of R. Soc. Edin._, vol. 24, pt. 6, p. 521). The Monotremata and
Marsupialia have curious Y-shaped spleens. As a rule flesh-eating
animals have larger and more notched spleens than vegetable feeders,
though among the Cetacea the spleen is relatively very small.
[Illustrtion: FIG. 2.--Section of the Spleen seen under a low power.
A, Fibrous capsule. d, Blood-vessels.
b, Trabeculae. e, Spleen pulp.
c, Malpighian corpuscles.]