The =lymphatic system= (Systema lymphaticum) is subsidiary to the venous
part of the circulatory system, from which it arises in the embryo. It
consists of the lymph vessels and glands.
The =lymph vessels= (Vasa lymphatica) contain a colorless fluid, the
lymph, which contains numerous lymphocytes.[175] They resemble the veins
in structure but have thinner walls and are provided with more numerous
valves. The vessels are sacculated opposite the segments of the valves
and have a characteristic beaded appearance when distended. The
collecting lymph vessels do not usually form rich plexuses, as veins
often do, their branching is more limited and less tree-like than that
of the blood-vessels, and their caliber therefore increases less from
the periphery toward their termination. All of the lymph is ultimately
carried into the venous system by two trunks, the =thoracic duct= and
the =right lymphatic duct=. Almost all of the lymph passes through at
least one group of lymph glands before entering the blood-vascular
system.
The =lymph glands= or =nodes= (Lymphoglandulæ) are intercalated in the
course of the lymph vessels. They vary widely in size, some being
microscopic, others several inches in length. In form they may be
globular, ovoid and flattened, elongated, or irregular. In certain
situations they are aggregated into groups, and a knowledge of the
position of these and the territory drained into them is important. It
is convenient, when possible, to indicate their position with regard to
arteries on the course of which they are placed. In color they are
usually gray or yellowish-brown in the dead subject, pink or
reddish-brown during life, but this varies according to their position
and functional state. The bronchial glands are often blackened by
infiltration with carbon. The mesenteric glands are creamy or white
while the chyle is passing through, but pink at other times. Vessels
which carry lymph to a gland are called =afferent=; the =efferent=
vessels which convey it away are larger and fewer. Each gland has a
depression, the =hilus=, at which the blood-vessels enter and the
efferent lymph vessels emerge.
=Lymph nodules= or =follicles= (Noduli lymphatici) are minute masses of
lymphoid tissue which occur in certain mucous membranes. They may be
solitary, as in the solitary glands of the intestine, or aggregated into
masses or patches, as in the tonsils and the so-called Peyer’s patches.
The =lymph nodule= or =follicle= is the unit of structure of the lymph
gland. It consists essentially of an artery surrounded by a
=reticulum= of connective tissue, the meshes of which contain numerous
=lymphocytes=. Surrounding this is a rich plexus of lymph vessels,
forming the so-called =sinus=, inclosed in some cases by a fibrous
capsule. The gland consists of a mass of follicles, inclosed in a
fibrous =capsule=, from which =trabeculæ= pass in and unite the
follicles. Beneath the capsule is the =peripheral sinus=, which
consists of a very rich plexus of lymph vessels; to this the afferent
vessels pass at various points of the surface. In the =cortical
substance= the cells are in rounded masses, the cortical nodules,
while in the =medullary substance= they lie around the arteries,
forming the so-called medullary cords. The =medulla= is redder than
the cortex, since it is more vascular; it contains the =central lymph
sinuses=, which have a similar structure to the peripheral sinus.
The =hæmolymph glands= differ from the lymph glands in color and
structure. They are of a deep red color, which is due to the high
vascularity of the cortical substance. The peripheral sinuses especially
are greatly developed and contain numerous red blood-cells. There is no
clear division into cortical and medullary substance, and the trabeculæ
contain smooth muscle-cells. Some have afferent and efferent lymph
vessels and others do not. They resemble the spleen in some respects,
but their significance is not yet clear. They are numerous in the ox and
sheep, much fewer in the dog, and apparently are absent in the horse.
They occur along the course of the aorta, in the perineal fat, at the
portal fissure, and with the gastric and mesenteric lymph glands. In the
ox they are also found under the trapezius muscle, under the skin of the
upper part of the flank, and in other places less constantly.
The =tissue= or =lymph-spaces= are interstices of varying size between
cells or in the meshes of connective tissue. They contain a fluid
derived from the blood-plasma, which is usually called =lymph=. They are
drained by the veins and lymph vessels. The large serous sacs are often
included in this category.
The exact relationship between the lymphatic capillaries and the
tissue spaces is still a matter of controversy. It is held by some
that the lymph vessels are in direct communication with the tissue
spaces, while others maintain that the lymphatics are complete closed
tubes. Communication between the spaces and vessels is in general very
free. Mall has shown that granules injected into the hepatic artery
are returned by the lymphatics as well as by the veins, and
intramuscular injections will enter the lymph vessels of the tendon in
spite of the absence of lymphatics in muscle.