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crossed by the pancreas and splenic vessels. Sometimes these structures lie at lower level, when the whole antero-lateral surface is covered by peritoneum of the omental bursa.

Blood and Lymph Vessels.-Typically, each gland receives three arteries : one direct froz the aorta, one from the inferior phrenic, and one from the renal artery; and is drained by of vein, which emerges at the hilum, the right to join the vena cava inferior, the left to join tileft renal vein. Numerous lymph vessels pass from the suprarenal glands to the lateral aortis lymph glands.

Nerves.—The nerves passing to and from the glands constitute the suprarenal plexue They connect with the renal and celiac plexuses and with the cæliac ganglia, and incluinumerous fibres from the greater splanchnic nerves, with a smaller number from the vagus ark. phrenic nerves. Most of them are medullated, but lose their sheaths on passing into sm ganglia in, or just within, the fibrous capsule of the gland. Thereafter they pass to the chromaphil tissue of the medulla, either directly, or after first supplying the cortex.

Structure.-A suprarenal gland consists of a highly vascular central mass of chromaphi tissue, the medulla, enclosed within a thick capsule of cortical substance, the cortes, which is turn is enclosed within a capsule of fibrous tissue, tunica fibrosa. From the deep aspect of the fibrous tunic trabeculæ pass inwards to support the glandular tissue. In the superficial part o the cortex the trabecula interlace freely to enclose rounded loculi, zona glomerulosa ; in the intermediate part they run vertically to the surface to enclose columnar spaces, zona fasciculata in the deepest part of the cortex they become broken up and form a reticulum, zona reticulata.

The cortex consists of polyhedral cells arranged in the interstices of the fibrous trabecul These cells contain a lipoid substance, which is present in sufficient quantity to give the cortes. yellow colour.

The medulla is formed of a spongework of cell columns bounding anastomosing venous sinuses. The cells are large, contain numerous granules, and possess the specific chromap. reaction. In a fresh gland the medulla is of a dark red colour owing to the presence of blood is its sinuses.

The blood vessels enter at numerous points in the fibrous capsule and run in the trabecula. forming a network around the cell masses and columns of the zona glomerulosa and zue. fasciculata. In the zona reticulata the blood vessels open up to form a venous plexus, which : continued through large sinuses in the medulla to reach a central vein. This is the tru which emerges at the hilus.

Development of the Cortical System and of the Suprarenal Glands. The cortical syste! is a derivative of the cælomic epithelium (mesoderm). The first indication of its developme!! is given, when the embryo is about 6 mm. in length, by the rapid proliferation of the epithelia cells placed between the mesonephros and the root of the mesentery. Numerous buds form a' penetrate the mesenchyme at the sides of and ventral to the aorta. In an 8-mm. embryo this buds have already lost their connexion with the cælomic epithelium. By the time the embro is 9 mm. long the developing cortical masses are vascularised. In man the greater part of thtissue thus formed is ultimately included in the cortex of the suprarenal glands, but small mass may escape, either at this stage or subsequently, to form independent cortical bodies.

In 12-min. embryos the developing suprarenal glands lie in a caudalward continuation of to dorsal portion of the pleuro-peritoneal membrane called the suprarenal ridge, and are composti. cortical tissue only. When the embryo is about 20 mm. in length sympatho-chromaphila destined to form the medulla of the gland, begin to migrate into the developing cortex. T: two kinds of tissue are in contact in 10-12 mm. embryos, but penetration of cortical master to sympatho-chromaphil cells has not been observed earlier than the stage mentioned.

At first the immigrating cells are scattered in numerous columns and strands, and it is L: until the embryo is about 10 cm. in length that they begin to reach the central vein and to form. true medulla. When the process of immigration ceases is unknown, probably not until after birth

The final specialisation of the cortex is a late phenomenon, and does not take place until lorg after birth. The zona reticularis develops early and is recognisable in a 15-min. embryo; to zona glomerulosa is not formed until the second or third year, but is represented until then sa layer of small incompletely specialised cells immediately under the fibrous capsule.

During fætal life the cortex is relatively enormous. This is due to a great proliferation the cells of the fætal zona reticularis, which differ from the adult cells of the same zone in D** containing lipoids. This fætal cortex begins to undergo a fatty degeneration soon after bir: and by the end of the first year has disappeared. The new cortex which replaces it devel? from the small, superficially placed, incompletely specialised cells already referred to.

Comparative Anatomy of the Chromaphil and Cortical Systems.-A knowledge of :) main facts of the comparative anatomy of the chromaphil system is a help to understanding it. distribution in man. Chromaphil tissue is first recognised with certainty in the cyclostomata, .. which it is arranged in thin strips on the walls of the larger arteries and their branches 1: elasmobranchs chromaphil bodies are present and are arranged segmentally on branches : the aorta in close relation to the ganglia of the sympathetic chain. Cortical tissue is also ir cognisable in the cyclostomata, in which it is arranged in small lobulated masses in the wall: the posterior cardinal veins and renal arteries. Even in this rudimentary form it is rub. lipoids. In the rays (elasmobranchs) the cortical system is represented by a pair of yellem coloured, rod-shaped structures in the region of the kidney. In batrachians the bromaphie cortical representatives first begin to come together. In the frog the adrenals are golden yeli

e ventral surface of the kidney. The greater part of these are made up of columns ls, but at the borders or ends of the cell columns masses of chromaphil cells occur. nent is transitional between the complete independence of the chromaphil and cortical lasmobranchs and the partial inclusion of chromaphil tissue within cortical charhe higher mammals (Swale Vincent). But even in man the union of the systems is plete. All the chromaphil tissue except the medulla of the suprarenal gland lies 1 with cortical substance. It appears therefore (i.) that the paraganglia of the trunk are homologous with the segmental chromaphil bodies of elasmobranchs ; (ii.) omaphil.bodies of the abdominal plexuses are a new formation confined to the higher

(iii.) that the inclusion of chromaphil tissue within a capsule of cortical tissue, as renal glands, is a still later development confined to the highest classes of animals.

THE DUCTLESS GLANDS OF EN TODERMAL ORIGIN. up of ductless glands, (i.) the thyreoid gland, (ii.) the parathyreoid i.) the thymus, and two pairs of inconstant, apparently functionless , (iv.) the cervical thymus glands, and (v.) the ultimo-branchial bodies, ped from the entodermal lining of the embryonic pharynx.

(i.) GLANDULA THYREOIDEA. hyreoid gland (O.T. thyroid body) is placed in the neck, firmly bound s tissue to the anterior and lateral aspects of the upper part of the ind to the sides of ix. It is yellowish

Thyreoid cartilage , and vascular. size with age, sex,

Crico-thyreoid eral nutrition, being

ligament y large in youth, in and in the well nour

Superior thyreoid

vein In women it increases rily with menstruaad pregnancy Its

Cricoid cartilage dimensions are, height

Internal jugular vein readth 6 cm., weight - but these measure

Isthmus of thyreoid

gland are of little value be

Left lobe of f the range of varia

thyreoid gland





wentionally, the thyreoid

Common s said to consist of two

carotid artery lobes united ddle line by a narrow of gland tissue, the

Inferior 5. To many thyreoids

thyreoid vein scription is inapplicable. and thin elderly spinsters and is not uncommonly shoe shaped ; in young Innominate artery ourished women and in Fig. 1058.- DISSECTION OF THE THYREOID GLAND AND OF THE PARTS -ncy its general contour ts a sphere, deeply notched orly to accommodate the larynx and deeply grooved posteriorly for the trachea and agus. Rarely, the gland is in two parts. Not infrequently, it is asymmetrical. out 40 per cent of specimens a process of gland tissue, the pyramidal lobe, extends the upper border of the isthmus, upwards, in front of the cricoid and thyreoid ges, towards the hyoid bone. This process is seldom median, lying more often on eft than on the right. In rare cases, it is double. Less rarely, it is double below ingle above. Sometimes it is represented by a strip of fibrous tissue or a narrow le (lig, suspensorium, or m. levator, glandulae thyreoidea). mall oval accessory thyreoid glands are common in the region of the hyoid bone, ure occasionally met with in relation to the right and left lobes. he relations of the gland are variable, depending upon its size and its relative level.

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In a majority of cases the isthmus covers the second, third, and fourth rings of th: trachea, but it may cover the cricoid cartilage, or the fourth, fifth, and sixth tracheal rings. Anteriorly, the gland is clothed by the pretracheal fascia, which separates it frou. the sterno-thyreoid, sterno-hyoid, and omo-hyoid muscles. Extensions of this fascial layer form an indefinite capsule for the gland. Postero-medially, the thyreoid gland is mouldei by the sides of the trachea and lower part of the larynx, and, when large, comes int contact, behind them, with the pharynx and @sophagus. Postero-laterally, it is in relation to the common carotid arteries, and when large is in intimate relation to the recurrent nerves. Further, it has important relations to the parathyreoid glands 'sert Parathyreoid Glands, Relations).

Blood and Lymph Vessels. — The blood supply is effected through the superior thyroid, arteries, branches of the external carotids, and through the inferior thyreoid arteries, branche of the thyreo-cervical trunks. Occasionally a fifth artery is present, the thyreoidea ima, branch of the innominate. The pyramidal lobe, if well developed, receives a special branch , from one of the superior thyreoids, usually the left. These arteries are remarkable for the large size and for the frequence and freedom of their anastomoses. An anastomosing trunk | courses up the posterior aspect of each lateral lobe, uniting the inferior and superior thyrrol arteries. It is of interest in connexion with the recognition of the parathyreoid glands Typically, three pairs of veins drain the gland. The upper two pairs, the superior and mid! thyreoid veins, join the internal jugulars; the lower pair, the inferior thyreoid veins, join the left innominate. These vessels take origin from a venous plexus on the surface of the glande. in the case of the inferior, from a downward extension of the plexus in front of the trachea When the gland is very large, accessory veins are present, sometimes in considerable numbes Most of these pass to the internal jugulars. A free, transverse, venous anastomosis is effecte i along the borders of the isthmus through the superior and inferior communicating veins.

The lymph vessels anastomose freely in the substance and on the surface of the gland. Most pass direct to the deep cervical lymph glands, a few descend in front of the trachea to il pretracheal lymph glands.

Nerves. — The nerves are derived from the middle and inferior cervical ganglia of the sympathetic. They accompany the blood-vessels.

Structure.—The gland is enclosed in a fibrous capsule (tunica propria which sen prolongations inward to form a frainework for the gland tissue proper. This consists of spheroidal vesicles, -04 mm. to 1 mm. in diameter, lined with cubical epithelium, and filled wil “colloid.” The size, shape, and cellular characters of the vesicles vary with diet and envird. ment. The vesicles are surrounded by networks of blood capillaries and of lymph vessels.

Development. The thyreoid gland takes origin from a single median outgrowth fre the pharyngeal floor (entoderm). It is recognisable as a shallow bay in a 1.8-mm. embry. practically simultaneously with the demarkation of the foregut. As the bud grows its : expands whilst its stalk narrows to form the thyreo-glossal duct.

In a 4-mm. embryo an elevation is present round the pharyngeal opening of the duct. To is the tuberculum impar. It migrates forwards, the duct backwards, so that in a 5-mm. emiti' the duct opens into the furrow immediately caudal to the tuberculum (see Tongue, Development At about this stage the duct begins to obliterate. This process proceeds slowly and is sebica quite complete, a vestige of the duct, the foramen cæcum of the tongue, remaining in the aiult.

While these processes are proceeding growth and lateral expansion of the bud continue. 1: becomes bilobed and has a divided lumen, and all the while it undergoes a continuous relatie displacement caudalwards. Soon its lumen disappears. In a 9-mm. embryo the developes thyreoid gland is a transverse bar composed of transversely disposed cell columns. At aboui ! tenth week of development, 55-mm. embryo, the formation of vesicles commences but is not exifpleted until after birth. The remaining changes are due to simple growth and the moulding effects of the pressure of surrounding structures.

The thyreoid gland does not arise in any of its parts from any of the pharyngeal rond (see Ultimo-branchial Bodies).

The developmental history of the gland affords a ready explanation of its variations in ta adult. Thus the development of a pyramidal lobe and its variations, partial and comp.: duplication, are due to the development of gland tissue from that part of the thyreo-glossal or which has a double lumen and the more or less complete fusion or separation of the masses t.. formed. Accessory thyreoid glands near the hyoid bone are the result of a similar primers connexion with isolated remnants of the duct.

The occurrence in the adult of a duct leading from the foramen cæcum to, or towaris, c. hyoid bone (lingual duct) is due to a persistence of the upper part of the thyreo-glossal du Similarly, thyreo-glossal cysts are due to the persistence of short intermediate lengths of the di..

(ii.) GLANDULE PARATHYREOIDEÆ. The parathyreoid glands (O.T. parathyroid bodies ; Synonyms, epithel bodies, parathymic glands, branchiogenic glands) are finely granular, yellow:brown, lenticular or spheroidal structures, from 2 to 20 mm. in diameter and fr.“

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1 weight. Most commonly they are lenticular, 5-7 mm. in length,
kness, and from .01 to 1 gm. in weight. Normally there are two pairs
Is, distinguished by the Roman numerals IV. and III. to signify that
com the fourth and third pharyngeal pouches. Sometimes, in course
t, the parathyreoid buds divide so that more than four, five to twelve,
e present: the numerals are then applied to the groups of glands
he pouches.
id IV. is commonly embedded in the tunica propria of the thyreoid gland
ior to the corresponding lateral lobe of that organ, about its middle. Para-
similarly embedded, usually lies on the posterior aspect of the inferior ex-
; lateral lobe of the thyreoid gland. As a rule the anastomosing channel,
ts the inferior and superior thyreoid arteries (see Thyreoid Gland, Blood
s near both parathyreoids and furnishes the best guide to their discovery, but
the exceptional positions which the glands may occupy is wide. Thus, para-
may be found (1) behind the pharynx or esophagus, (2) in the fibrous tissue
i the larynx, above the level of the thyreoid gland, (3) behind any part of the
globe of the thyreoid gland or even embedded in the thyreoid substance
athyreoid); whereas parathyreoid III. may be found (1) near the bifurcation
mon carotid artery, (2) behind any part of the corresponding lobe of the
ind, (3) on the
trachea, or (4)
vessels. - The
y of each para-

Left lobe of thyreoid effected by a

gland ry which enters

Isthmus of thyreoid at its hilum. It

gland s from any branch

Internal jugular vein erior or superior rteries, but most Vagus nerve

Common carotid artery is a branch of the Common carotid artery

Internal jugular vein tomosing channel Innominate artery

Band connecting ferred to.

thymus with thyreoid

Vagus nerve ture. - The para- Subclavian artery

Subclavian artery are built up of in

Subclavian vein anicating trabeculæ subclavian vein helial cells with

Innominate vein of vascular connecsue between them. s are of two kinds, I, the other, the mincontaining oxyphyl s. Sometimes they nd spaces recalling d vesicles but there ormation of " colloid,"

HY MUS possibly after thyctomy. evelopment. — The ayreoid bodies develop the dorsal diverticula he third and fourth yngeal pouches. The indication of their de

PERICARDIUM pment is a proliferation thickening of the epiium on the cranial and ral

aspects of the diverLla. This is present in h the third and fourth aches in 9-10 mm. embryos

Fig 1059.—THYMUS IN A FULL-TIME FETUS HARDENED BY t appears to be rather

FORMALIN-INJECTION. egular in the time of its pearance. The cells forming it are vacuolated, difficult to stain, and indistinct in outline. ords of cells grow out from the thickening and fibrous tissue penetrates between the outowing cords, which soon lose their connexion with the pharynx. The differentiation of the vo kinds of cells takes place after birth.

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