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5-17 gm.

Groove for left

vena cava

surface

e to say what should be the normal dimensions of the gland at the In some new-born babes it weighs as little as 2 or 3 gm., in others

At puberty it may be difficult to find, or may weigh as gm. After the age of fifty it may require careful dissection to dis

be quite large. When large it fills all the space available between acs laterally, the back of the sternum in front, and the pericardium ssels behind; when small it is embedded in fat and fibrous tissue. f the gland varies with its size and the age of the individual. In

short thoraces it is broad and squat; in adults with long thoraces out into two finger-like strands. The details of its shape are deters size and by the structures upon which it is moulded, viz., the perid the great vessels of the superior mediastinum and the root of the

or both of its lobes may be connected by a strand of fibrous tissue ca propria of the thyreoid gland. d Lymph Vessels. —The blood supply of the thymus is effected through inconstant the inferior thyreoid and internal mammary arteries. Its veins are irregular and ferior thyreoid, internal mammary, and innominate veins. Its lymph vessels are ass to glands close to the organ. res are minute and are derived from the vagus and sympathetic. The branches of lescend directly to the thymus from about the level of the thyreoid cartilage; the ic fibres run with the blood-vessels. The fibrous capsule of the thymus receives small ranches from the phrenic nerves, but these do not supply the gland tissue in any way. ure. — The thymus is invested by a

Groove for -psule which sends septa into its sub- pulmonary artery

innominate vein divide it into lobules. The lobules are

Mediastinal

Groove for divided into follicles (secondary lobules) mm. in diameter. Each follicle consists

superior

Mediastina culla not quite completely surrounded by - In a general way, the structure of the esembles a lymph gland, but the reticulum, of being fibrous, is syncytial. The spaces ceticulum are crowded with lymphocytes, =re are no germinal centres. The medulla les the cortex, but the reticulum is coarser ntains cell nests, the concentric corpuscles ssall. As age advances the thymus under

Pericardial surface nvolution. This process is marked by an Fig. 1061.- DEEP SURFACE OF Thymus, TAKEN use of fibrous tissue and a reduced cellularity.

A FETUS HARDENED BY FORMALINmumber of lymphocytes and of concentric scles varies with nutrition. Development.--As has been stated, there are in reality two thymus glands, a right and a

they arise from the ventral diverticula of the third pharyngeal pouches. The first cations of the developing glands, cylindrical elongations of the diverticula, are present in m. embryos; the walls of the cylinders, more particularly their dorsal parts, soon thicken. ncidently the necks of the pharyngeal pouches become constricted to form the pharyngonchial ducts III. These soon disappear when the thymus rudiments lose all connexion Eh the pharynx. At this time the upper parts of the rudiments still have a lumen; the ver parts are solid. Soon the lumen vanishes; the solid parts thicken and the developing ymus migrates caudalwards to reach the pericardium at the 15-mm. stage. As a result of the igration the upper part gets drawn out and finally disappears. It is in this process that rathyreoid III. is involved. It is attached to the upper part of the migrating thymus, the part hich disappears. The relative time of this disappearance determines the permanent level of arathyreoid 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 besido 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

[graphic]

FROM
INJECTION.

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 relatiou 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 ti 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, miinly 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 tetrabedral 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,

| renal surfaces; anterior, at the junction of the diaphragmatic, gastric, and ces ; posterior, at the junction of the diaphragmatic, renal, and basal surfaces ; te, at the junction of the renal, gastric, and basal surfaces. In a spleen of ment form there are but two angles, a superior and an anterior. The superior d in the same way as in the tetrahedral form; the anterior, by the diaic, gastric, and renal surfaces. In all spleens, but most commonly in those of irregularly tetrahedral form, the superior angle may curve forward as a blunt

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

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Fig. 1062.-DISSECTION OF THE SPLEEN, LIVER, AND KIDNEYS FROM BEHIND, IN A SUBJECT HARDENED BY

FORMALIN-INJECTION.

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 trabeculæ only, not from the glandular part of the organ.

Nerves.—The nerves are almost entirely non-medullated and come from the cæliac 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 tissue but includes many elastic and muscle fibres. From its deep surface, more especially at the hilum, strong trabeculæ pass into the organ to support the blood-vessels and nerves.

ANTERIOR BORDER

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Posterior angle
Fig. 1063.—THE SPLEEN-VISCERAL ASPECT.
The pulp is a spongework of fine fibres covered, at places entirely concealed, by branched
connective-tissue corpuscles, reticulum cells. Associated with these, occupying some of the

Central blood-space

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Central blood-space Fig. 1064.—SECTION THROUGH Glomus COCCYGEUM (highly magnified). From J. W. Thomson Walker. smaller spaces of the spongework, are cells like very large leucocytes, spleen phagocytes. These are ameboid and often contain the debris of red blood corpuscles.

Arterial branches

es,

s enter at the hilum, run in the trabeculæ, and branch freely. The smaller Lymphoid sheath developed in their walls. This replaces the fibrous sheath which eries receive from the trabeculæ. Every here and there the lymphoid sheath netrically or asymmetrically to form a lymphatic nodule (nodulus lymphaticus ay of the nodules thus formed are quite small; others are visible to the naked eye is, but, however large or small they may be, each contains a network of capillaries. r termination the arteries lose their sheaths and become reduced to simple tubes of ells; gaps appear in their walls and finally the cells forming them ecome con

the reticulum cells of the pulp. The veins begin in the same way as the arteries ulp is, therefore, the modified capillary system of the spleen. ment. —The spleen is mesodermal in origin. The first indication of its develop)-mm. embryo, is a thickening of the dorsal mesogastrium. In 10-12 mm. embryos ; epithelium over the splenic rudiment is several layers thick. Soon the deeper e thickening are transformed into mesenchyme and the epithelium is reduced again layer. The first vascularisation of the spleen is effected by a capillary network. Out intra-splenic arteries and veins te. The undifferentiated capilveen them form capillary tufts

These become transformed pulp. The exact method of this ation is undetermined, but during ess great numbers of red blood

Arterial branches produced. The lymphatic nodules loped in the later part of fætal with their contained lymphocytes tiate from the tunica adventitia of ries.

Accessory glomus

Accessory

glomus ii.) Glomus COCCYGEUM.

Accessory glomus. ne glomus coccygeum is a

body, 2-2.5 mm. in diameter, d immediately anterior to the tip e coccyx, upon a branch of the Ile sacral artery.

Usually it is Entrance of artery

into main glomus mpanied by a group of smaller

Accessory ies of similar structure and arterial

glomus tion.

Main glomus-

Structure. — The glomus is enclosed Accessory

glomus a fibrous capsule and consists of round polyhedral cells with large nuclei lining considerable blood space, which is an nastomosing channel between an artery

FIG. 1065.-SCHEMA OF THE RELATION PRESENTED BY THE nd a vein.

GLOMUS CoccYGEUM AND ITS ACCESSORY OUTLYING

PARTS TO THE BRANCHES OF THE MIDDLE SACRAL Development. The glomus develops

ARTERY. (Reconstructed from serial sections through rom the capillary network of the region

the region.) From J. W. Thomson Walker. of the tip of the embryonic tail. At first

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).

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