are arranged concentrically at the periphery; and in the center of the nodule the reticular tissue usually has wider meshes, and the lymph-cells are less densely placed. (Fig. 153.) In the center of the nodule the cells often show numerous mitoses, and it is here that an active proliferation of the cells takes place. The cells may either remain in the lymph-follicle or the newly formed cells are pushed to the periphery of the nodule, and are then swept into the circulation by the slow lymph current which circulates between the wide meshes of the reticular connective tissue. Flemming (85, II) has called that central part of the nodule containing the proliferating cells the germ center or secondary nodule (compare p. 175). The germ centers are transitory structures, and are consequently found in different stages of development. They may even be absent for a time.

Fig. 153. A solitary lymph-nodule from the human colon. At a is seen the pronounced concentric arrangement of the lymph-cells.

The lymph-glands are organs of a more complicated structure, but also consist of lymphoid tissue. They are situated here and there in the course of the lymph-vessel and are widely distributed. Their size varies greatly. In shape they are much like a bean or kidney, and the indentation on one side is known as the hilum. The afferent lymph-vessels, the vasa afferentia, enter at the convex surface. of the organ, while the efferent vessels, the vasa efferentia, pass out at the hilum. The whole gland is surrounded by a capsule consisting of two layers: the outer is made up of a loose, and the inner of a more compact, connective tissue in which a few smooth musclefibers are imbedded. Portions of the inner layer pass into the substance of the gland to form septa, or trabecule, by means of which the organ is divided into a number of imperfectly separated compart

ments.

The lymphoid substance of the gland is so distributed that at its periphery a large number of lymph-nodules are placed in dense masses separated from each other by the trabeculæ just described, the cortical nodules. The nodules are identical in structure with those mentioned above. They form a peripheral layer which is, however, not clearly defined in the neighborhood of the hilum. This layer is known as the cortex of the lymph-gland. (Fig. 154.) The lymphoid tissue of the interior of the gland, the medullary substance, is in the shape of cords-medullary cords. These connect with each other and form a network of lymphoid tissue, in the open spaces of which lie the trabeculæ. At their periphery the nodules and medullary cords gradually pass into a wide-meshed lymphatic tissue, the lymph-sinus of the gland, parts of which lie (1) between the capsule and the cortical substance, (2) between the nodules of the latter and the trabeculæ, (3) between the medullary cords and the trabeculæ, and (4) between the medul

Fig. 154.-Section through a mesenteric lymph-gland of cat, with injected blood-vessels; X 50: a, Medullary substance; b, cortical substance with cortical nodules.

lary substance and the capsule at the hilum. The sinus is therefore intimately connected not only with the capsule, but also with the trabeculæ. At the hilum the loose lymphoid tissue represents a terminal sinus (Toldt).

The inner wall of the capsule and the trabecula with their processes are covered by flattened endothelial cells which are continuous with those of the afferent and efferent lymph-vessels. The lymph flows into the gland through the afferent vessels, and passes along into the interior through the spaces offering the least resistance (sinuses). The latter represent those peripheral portions of the nodules and of the medullary cords in which the lymphoid tissue is present in loose arrangement. The lymph consequently envelops not only the lymph-nodules of the cortical substance, but also the medullary cords, and finally streams into the terminal sinus and

then into the efferent channels. As a result the lymph takes with it the newly formed cells of the lymph-nodules and the medullary cords, and passes out much richer in cellular elements than on its

entrance.

A large number of arterial blood-vessels enter the lymph-gland through the hilum and penetrate into the interior of the organ through the trabeculæ. After passing through the sinuses they break up into capillaries in the medullary cords or in the lymphnodules of the cortical substance. The sinuses, then, contain no

Fig. 155. From a human lymph-gland; X 240. At a are seen the concentrically arranged cells of the lymph-nodules. (Fixation with Flemming's fluid.)

capillaries. The arterial capillaries pass over into the venous capillaries, and the veins resulting from the union of the latter pass to the periphery of the organ side by side with the arteries.

C. THE SPLEEN.

The spleen is a blood-forming organ, in which white blood-cells and, in embryonic life and under certain conditions in adult life also, red blood-cells are formed-the former in the adenoid tissue (Malpighian corpuscles) and spleen pulp, the latter only in the spleen pulp.

The spleen is covered by peritoneum, and possesses a capsule

consisting of connective tissue, elastic fibers, and nonstriated musclecells. This capsule sends numerous processes or trabeculæ into the interior of the organ, which branch and form a framework in which the vessels, especially the veins, are imbedded. This connective-tissue framework breaks up to form the reticular tissue which constitutes the ground substance of the spleen.

On examining a section of the spleen with the low-power magnifying glass, sections of the trabeculæ, and round or oval masses of cells, having a diameter of about 0.5 mm., and in structure and

(Sublimate fixation.)

Fig. 156.-Part of a section through the human spleen; X 75.
At a is an oblong Malpighian body with a blood-vessel.

appearance similar to the lymph-nodules (Malpighian corpuscles), are clearly defined; between and around these structures is a tissue rich in cells, blood-vessels and blood-corpuscles, known as the spleen pulp.

The organ has a very typical blood supply. Its arteries enter at the hilum, or indented surface, and its veins pass out at the same place. On the penetration of the vessels through the capsule, the latter forms sheaths around them (trabecula), but as soon as the arteries and veins separate, the trabeculæ envelop the veins alone.

The arteries break up into smaller branches, which in turn divide into a large number of tuft-like groups of arterioles. Soon after their separation from the veins, the adventitia (outer fibrous tissue coat) of the arteries begins to assume a lymphoid character. This lymphoid tissue increases here and there to form true lymphoid nodules, possessing all the characteristics already mentioned-reticular tissue, germ centers, etc. These are the Malpighian bodies, or corpuscles; they are not very plentifully represented in man. The Malpighian bodies with their germ centers are formative centers for the lymphocytes. The newly formed cells pass into the pulp and mix with its elements, which are then bathed by the blood emptying from the arterial capillaries into the channels of the pulp. The lymphoid sheaths and nodules derive their blood supply from arteries which arise from the lateral branches of the splenic vessels, and which divide into capillaries inside of the lymph sheaths or nodules, and only assume a venous character outside of the lymphoid substance. These vessels constitute the nutritive vascular system of the spleen.

The small arterial branches above mentioned break up into very fine arterioles which gradually lose their lymphoid sheath, so that branches with a diameter of 0.02 mm. no longer possess a lymphoid sheath, but again assume an adventitia of the usual type. The smallest arterioles now pass over into capillaries which are for a time accompanied by the adventitia (capillary sheath), while the terminal branches have the usual structure of the capillary wall and are gradually lost in the meshes of the pulp. (See below.) On the other hand, the beginnings of the venous capillaries may be distinctly seen in the pulp spaces. Groups of these capillaries combine to form larger vessels, which, however, still retain a capillary structure, and these again form small veins which unite to form the larger veins.

F. P. Mall, whose recent contributions on the structure of the spleen have greatly extended our knowledge of the microscopic anatomy of this organ, states that the trabecular and vascular systems together outline masses of spleen pulp about 1 mm. in diameter, which he has named spleen lobules. Each lobule is bounded by three main interlobular trabeculæ, each of which sends three intralobular trabeculæ into the lobule which communicate with each other in such a manner as to divide the lobule into about ten smaller compartments. An artery enters at one end of the lobule and, passing up through its center, gives off a branch to the spleen pulp found in each of the ten compartments formed by the intralobular trabeculæ. The spleen pulp in these compartments is arranged in the form of anastomosing columns, or cords, to which Mall has given the name. of pulp cords. The branches of the main intralobular artery, going to each compartment, divide repeatedly; the terminal branches course in the spleen-pulp cords, and in their path give off numerous small side branches which end in small expansions known as the ampullæ of Thoma. "The first two-thirds of the ampulla are lined