Closely associated with the lymphatic capillaries and vessels is a group of glandular structures known as lymphoid organs. They consist, essentially, of groups of round lymphoid cells, lying in a meshwork of reticulum fibers, and having often a definite relationship to the blood or lymph vessels.
The group of lymphoid organs includes, in addition to the lymph-glands [lymphoglandulse] or lymph-nodes, which are particularly related to the lymphatic vessels, the spleen, thymus and bone-marrow, which are also largely made up of lymphoid tissue. The spleen and thymus, however, are considered separately with the Ductless Glands.
In their most simple form, the lymphoid organs form mere irregular accumulations or patches of lymphoid cells, which have been termed lymphoid infiltrations. Such patches are frequent in mucous membranes especially along the intestinal tract and the air-passages in the lungs.
Larger accumulations of lymphoid cells produce definite round nodules, which may occur singly, as solitary follicles or in groups, as aggregated follicles (Peyer's patches). In the solitary follicle the lymphoid cells are arranged concentrically, with a region in the center where the cells are less closely packed together. This is called the germinal center, and contains numerous cells undergoing mitotic division. The solitary follicle contains blood-capillaries. Lymph-capillaries, however, do not enter the follicle but form a rich plexus about it.
The lymph-glands or nodes are larger lymphoid structures, which are developed along the course of the lymph-vessels. They vary much in size, shape, and color, and may occur singly or in small or large groups. The size varies from the size of a pin-head to that of an olive, or larger. In shape they may be spherical, oval, or flattened on one or more sides, according to their relations to other organs. Each gland has an indentation or hilus, where the arteries enter, and where the veins and efferent ducts emerge. Their color depends upon position and state of function. The glands along the respiratory tract are black, due to the presence of carbon granules. The mesenteric glands are milk-white during digestion, and other nodes are pale and translucent when their sinuses are filled with fluid, and pink or even red when red-blood cells are present in the sinuses. The lymph-gland is made up of four distinct elements: lymphoid elements, lymphatic capillaries, supporting structures, and blood-vessels.
The lymphoid elements are arranged as follicles and as cell-strings. The follicles lie around the circumference of the gland, and form the cortex [substantia corticalis]. The cell strings or medullary cords are irregular cords of cells which extend from the follicles through the central or medullary portion [substantia medullaris] of the gland. The follicles and medullary cords are made up, as are the solitary follicles, of round lymphoid cells.
The lymphatic vessels enter the lymph-gland as several vasa afferentia, and leave it, at the hilus, as the vasa efferentia. The vasa afferentia spread out in the cortical portion of the gland into an extremely rich plexus of wide capillaries which surround the follicles, forming the peripheral sinus. The capillaries do not enter the follicle. This plexus continues, around the follicles, into the medullary portion where it forms again a rich plexus, the medullary sinus, in the spaces around the medullary cords. At the hilus the medullary capillaries collect into larger vessels and emerge as the vasa efferentia.
The supporting structures consist of a fibrous capsule surrounding the gland, from which trabecula3 or septa pass in, around and between the follicles and cords. From the septa, a fine reticulum passes into the follicles and cords, where it forms a rich dense meshwork, in the interstices of which lie the Iymphoid cells. The capsule and trabeculae are made up of white fibers, elastic fibers and smooth muscle-fibers.
The blood-vessels, which enter and leave at the hilus, send branches into the follicles and into the medullary cords.
The enormous widening of the lymph-stream in the lymph-node from the vasa afferentia to the capillaries - like a brook widening out into a pond - causes a very great diminution in the rate of flow of the lymph. Thus there is present in the gland a very slowly moving stream of lymph, which is separated from the lymphoid tissue outside by a single layer of flattened endothelial cells. There is thus possible an easy interchange of substances, and an opportunity for the passage, through the endothelium, of wandering cells. While the entire mode of functioning of the lymph-gland is not clear, it is known that lymphocytes, formed here, enter the lymph-stream, and that substances such as, for instance, carbon granules, or leucocytes laden with bacteria, are checked in their course by the lymph-gland.
The lymph-glands are so arranged throughout the body that all the lymph which enters the lymphatic capillaries must pass through one or more lymph-glands on its way to the veins.
It is possible that this rule may have exceptions, although none have yet been definitely proved. Thus, some of the small lymphatics which join the thoracic duct may enter it without having passed through a gland. Moreover, there is often found a direct anastomosis between an afferent and an efferent lymphatic vessel.
Most of the glands are collected in certain regions, where they form centers toward which the lymphatic vessels radiate. Such groups are termed regional glands. The glands forming such a group are connected with one another by numerous anastomoses, which are termed lymphatic plexuses [plexus lymphatici]. In addition to the regional glands there are many isolated glands which lie along the course of the lymph-vessels, and through which pass the vessels draining a much more limited capillary area. Such glands are termed intercalated glands.
From Morris's treatise on anatomy.