5 is impossible to say what should be the normal dimensions of the gland at the rious ages. In some new-born babes it weighs as little as 2 or 3 gm., in others = much as 15-17 gm. At puberty it may be difficult to find, or may weigh as uch as 40 gm. After the age of fifty it may require careful dissection to disover, or may be quite large. When large it fills all the space available between he pleural sacs laterally, the back of the sternum in front, and the pericardium and great vessels behind; when small it is embedded in fat and fibrous tissue. The shape of the gland varies with its size and the age of the individual. In fants with short thoraces it is broad and squat; in adults with long thoraces is drawn out into two finger-like strands. The details of its shape are deterined by its size and by the structures upon which it is moulded, viz., the periardium and the great vessels of the superior mediastinum and the root of the eck. One or both of its lobes may be connected by a strand of fibrous tissue o the tunica propria of the thyreoid gland.

Blood and Lymph Vessels. The blood supply of the thymus is effected through inconstant branches of the inferior thyreoid and internal mammary arteries. Its veins are irregular and oin the inferior thyreoid, internal mammary, and innominate veins. Its lymph vessels are Large and pass to glands close to the organ.

Its nerves are minute and are derived from the vagus and sympathetic. The branches of the vagus descend directly to the thymus from about the level of the thyreoid cartilage; the sympathetic fibres run with the blood-vessels. The fibrous capsule of the thymus receives small irregular branches from the phrenic nerves, but these do not supply the gland tissue in any way.

Štructure. The thymus is invested by a fibrous capsule which sends septa into its substance to divide it into lobules. The lobules are similarly divided into follicles (secondary lobules) about 15 mm. in diameter. Each follicle consists of a medulla not quite completely surrounded by a cortex. In a general way, the structure of the cortex resembles a lymph gland, but the reticulum, instead of being fibrous, is syncytial. The spaces of the reticulum are crowded with lymphocytes, but there are no germinal centres. The medulla resembles the cortex, but the reticulum is coarser and contains cell nests, the concentric corpuscles of Hassall. As age advances the thymus undergoes involution. This process is marked by an FIG. 1061.-DEEP SURFACE OF THYMUS, TAKEN increase of fibrous tissue and a reduced cellularity. FROM A FETUS HARDENED BY FORMALINThe number of lymphocytes and of concentric INJECTION. corpuscles varies with nutrition.

Pericardial surface

Groove for

pulmonary artery

Mediastinal
surface

Mediastina surface

Development. As has been stated, there are in reality two thymus glands, a right and a left; they arise from the ventral diverticula of the third pharyngeal pouches. The first indications of the developing glands, cylindrical elongations of the diverticula, are present in 5-mm. embryos; the walls of the cylinders, more particularly their dorsal parts, soon thicken. Coincidently the necks of the pharyngeal pouches become constricted to form the pharyngobranchial ducts III. These soon disappear when the thymus rudiments lose all connexion with the pharynx. At this time the upper parts of the rudiments still have a lumen; the lower parts are solid. Soon the lumen vanishes; the solid parts thicken and the developing thymus migrates caudalwards to reach the pericardium at the 15-mm. stage. As a result of the migration the upper part gets drawn out and finally disappears. It is in this process that parathyreoid III. is involved. It is attached to the upper part of the migrating thymus, the part which disappears. The relative time of this disappearance determines the permanent level of parathyreoid III., for until it happens that gland is dragged in the wake of the thymus (see Parathyreoids, Development of). Sometimes a small detached mass of thymus formative tissue may persist beside parathyreoid III., and may differentiate to form an Accessory Cervical Thymus III.

During migration and after, the cells continue to proliferate and the thymus rudiment increases in mass. At the 40-mm. stage lymphocytes begin to appear in it. Differentiation of cortex and medulla is visible at the 45 mm. stage. The details of the process of thymic histogenesis are undetermined. It is believed that the syncytial reticulum and concentric corpuscles are of entodermal origin, but whether the lymphocytes arise in situ or are immigrants is unknown.

(iv.) THE CERVICAL THYMUS VESTIGES.

Small masses of thymus tissue are frequently found in close relation to parathyreoids IV. They are developed from the ventral diverticula of the fourth pharyngeal pouches in a manner generally similar to that in which the main thymus gland develops. Not

infrequently they are embedded in the thyreoid gland, internal thymus. Less frequently they are sausage-shaped bodies, 5.25 mm. in length, lying at the sides of the trachea Sometimes they are long enough to extend into the thorax, where they come into relation with the main thymus gland. Rarely they migrate into the thorax and develop to form considerable portions of the main thymus gland, which then consists of four developmentally distinct parts. Cervical Thymus Vestiges IV. require to be distinguished from the Accessory Cervical Thymus Glands III., which are sometimes found in close relation to parathyreoids III. (see Thymus Gland, Development).

(v.) THE ULTIMO-BRANCHIAL BODIES.

The fifth pharyngeal pouches give rise to a pair of gland rudiments called ultimobranchial bodies. Their fate is not known; it is possible that they may become the strands of parathyreoid-like tissue occasionally present in the lateral lobes of the thyreoid more frequently, apparently, they degenerate and leave no vestiges.

Their chief interest is that, for long, they were erroneously regarded as the rudiments of the lateral lobes of the thyreoid gland.

3. THE DUCTLESS GLANDS ASSOCIATED WITH THE
VASCULAR SYSTEM.

Two ductless glands of mesodermal origin are associated with the vascular * system. They are (i.) the spleen, (ii.) the glomus coccygeum.

(i.) LIEN.

The spleen is a soft, highly-elastic, contractile organ of purplish colour, placed in the upper left posterior part of the abdominal cavity, partly in the epigastrium, mainly in the left hypochondrium. It is moulded by the diaphragm, kidney, stomach, and, sometimes, colon. Its dimensions vary widely, but on the average are: length, 12 cm. ; breadth, 7 cm.; thickness, 4 cm.; weight, 160 gm. Its shape is modified by the relaxation, distension, and contraction of the neighbouring hollow viscera; its position and the details of its moulding, by the attitude of the individual.

When the stomach is contracted and the colon distended, the spleen has the form of an irregular tetrahedron; when the conditions of the hollow viscera are reversed, the form of a segment of an orange. These are the extremes of a series of forms which the spleen presents when hardened in situ. In the recumbent posture the long axis of the spleen corresponds in direction with the posterior part of the tenth rib; in the erect attitude the long axis is frequently vertical, more especially in adult females.

Surfaces, Borders, and Angles of the Spleen.-The surfaces of the spleen are the facies diaphragmatica, facies renalis, facies gastrica, and, in the tetrahedral form only, facies basalis (colica). Their contour is fairly constant. The diaphragmatic surface is convex, moulded to the curve of the diaphragm; the renal is slightly concave, moulded to the kidney; the basal is flat or slightly concave, moulded by the colon; the gastric, deeply concave, moulded by the stomach. The gastric surface is interrupted by an irregular, not infrequently divided, slit, the hilum, through which the branches of the splenic artery enter and the tributaries of the splenic vein leave the gland. On the same surface, behind the hilum, there is a depression for the tail of the pancreas.

The borders of the spleen are margo anterior, between the gastric and diaphragmatic surfaces; margo posterior, between the diaphragmatic and renal surfaces; margo intermedius, between the renal and gastric surfaces. When present the basal surface is separated from the diaphragmatic by the margo inferior, from the gastric and renal by the margines intermedii, anterior and posterior.

The anterior border of the spleen is almost invariably notched; most frequently there are two notches, but there may be six or seven. Sometimes the posterior border also is notched. Rarely, the notches on the borders are joined by fissures extending across the diaphragmatic surface.

The angles of a tetrahedral spleen are: superior, at the junction of the diaphragmatic,

gastric, and renal surfaces; anterior, at the junction of the diaphragmatic, gastric, and basal surfaces; posterior, at the junction of the diaphragmatic, renal, and basal surfaces; intermediate, at the junction of the renal, gastric, and basal surfaces. In a spleen of orange-segment form there are but two angles, a superior and an anterior. The superior

is bounded in the same way as in the tetrahedral form; the anterior, by the diaphragmatic, gastric, and renal surfaces. In all spleens, but most commonly in those of oblique, irregularly tetrahedral form, the superior angle may curve forward as a blunt hook.

The spleen is entirely covered with peritoneum and is moored by two peritoneal folds, the lieno-renal and gastro-splenic ligaments (pp. 1162 and 1236). Inferiorly it is supported by the peritoneal phrenico-colic ligament (p. 1242).

Small globular accessory spleens are often present. As a rule they are attached to the gastro-splenic ligament near the splenic hilum.

Blood and Lymph Vessels.-The spleen receives its blood from the splenic artery, which passes through the lieno-renal ligament. Before reaching the gland it breaks up into six or more branches which enter the hilum independently. The vein of the spleen, the splenic vein, is formed in the lieno-renal ligament by the union of several unnamed tributaries which emerge from the hilum. The lymph vessels also leave the spleen at the hilum. They are small and come from the capsule and trabeculae only, not from the glandular part of the organ.

Nerves.-The nerves are almost entirely non-medullated and come from the cœliac plexus. They accompany the splenic artery and its branches.

Structure. The spleen is composed of a soft substance called pulp, supported by fibrous trabeculæ and enclosed within a fibrous capsule, tunica propria, which, in turn, is enclosed within a peritoneal capsule, tunica serosa. Embedded in the pulp are nodules of lymph tissue, noduli lymphatici lienales (O.T. Malpighian corpuscles).

The tunica serosa closely invests the organ, except where it is reflected on to the lieno-renal and gastro-splenic ligaments and at the hilum. It is very firmly bound to the tunica propria.

The tunica propria is stout and strong but highly elastic. It is composed mainly of fibrous

cludes many elastic and muscle fibres. From its deep surface, more especially at the 3 trabeculæ pass into the organ to support the blood-vessels and nerves.

ANTERIOR BORDER

a spongework of fine fibres covered, at places entirely concealed, by branched e corpuscles, reticulum cells. Associated with these, occupying some of the

Central blood-space

Capillaries

From J. W. Thomson Walker.

Central blood-space THROUGH GLOMUS COCCYGEUM (highly magnified). e spongework, are cells like very large leucocytes, spleen phagocytes. These ten contain the debris of red blood corpuscles.

The arteries ente

eries have a lymphe larger arteries re rands symmetrical alis. Many of th white specks, but, wards their term athelial cells

ous with the

The pulp Developmen at, in a 9-mu

Le

cœlomic ep
rers of the threer

a single layer. Th this the intra-sple ferentiate.

The

aries between them
spherules. These

the pulp. The
sformation is un
progress great

s are produced. developed in and with their ferentiate from arteries.

The arteries enter at the hilum, run in the trabeculae, and branch freely. The smaller teries have a lymphoid sheath developed in their walls. This replaces the fibrous sheath which e larger arteries receive from the trabeculae. Every here and there the lymphoid sheath pands symmetrically or asymmetrically to form a lymphatic nodule (nodulus lymphaticus nalis). Many of the nodules thus formed are quite small; others are visible to the naked eye white specks, but, however large or small they may be, each contains a network of capillaries. owards their termination the arteries lose their sheaths and become reduced to simple tubes of dothelial cells; gaps appear in their walls and finally the cells forming them become connuous with the reticulum cells of the pulp. The veins begin in the same way as the arteries nd. The pulp is, therefore, the modified capillary system of the spleen.

Out

Development.-The spleen is mesodermal in origin. The first indication of its developaent, in a 9-mm. embryo, is a thickening of the dorsal mesogastrium. In 10-12 mm. embryos he coelomic epithelium over the splenic rudiment is several layers thick. Soon the deeper ayers of the thickening are transformed into mesenchyme and the epithelium is reduced again o a single layer. The first vascularisation of the spleen is effected by a capillary network. of this the intra-splenic arteries and veins differentiate. The undifferentiated capillaries between them form capillary tufts or spherules. These become transformed into the pulp. The exact method of this transformation is undetermined, but during its progress great numbers of red blood cells are produced. The lymphatic nodules are developed in the later part of foetal life, and with their contained lymphocytes differentiate from the tunica adventitia of the arteries.

(ii.) GLOMUS COCCYGEUM.

The glomus coccygeum is a small body, 2-2.5 mm. in diameter, placed immediately anterior to the tip of the coccyx, upon a branch of the middle sacral artery. Usually it is Entrance of artery

into main glomus

accompanied by a group of smaller bodies of similar structure and arterial relation.

Structure.

The glomus is enclosed in a fibrous capsule and consists of round or polyhedral cells with large nuclei lining a considerable blood space, which is an anastomosing channel between an artery and a vein.

Arterial branches

FIG.

Accessory glomus

Accessory glomus.....

Accessory
glomus

Main glomus--

Accessory
glomus

Arterial branches

1065.-SCHEMA OF THE RELATION PRESENTED BY THE
GLOMUS COCCYGEUM AND ITS ACCESSORY OUTLYING
PARTS TO THE BRANCHES OF THE MIDDLE SACRAL
ARTERY. (Reconstructed from serial sections through
the region.) From J. W. Thomson Walker.

Development.-The glomus develops from the capillary network of the region of the tip of the embryonic tail. At first the capillary walls differentiate as if to form an artery, then the cells of the middle coat, instead of forming muscle, assume an epithelioid character.

The position of the glomus coccygeum at the posterior end of the axis of the body, and the fact that its blood-spaces form a wide arterio-venous anastomosis, suggest that it is a sort of safety-valve on the peripheral circulation. No evidence that it produces an internal secretion has, as yet, been obtained, and in spite of frequent statements to the contrary it contains no chromaphil cells (Stoerk).