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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 from the aorta, one from the inferior phrenic, and one from the renal artery; and is drained by or vein, which emerges at the hilum, the right to join the vena cava inferior, the left to join the left renal vein. Numerous lymph vessels pass from the suprarenal glands to the lateral aortic lymph glands.
Nerves. The nerves passing to and from the glands constitute the suprarenal plexuses They connect with the renal and coeliac plexuses and with the coeliac ganglia, and includ numerous fibres from the greater splanchnic nerves, with a smaller number from the vagus and 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 cortex, which in turn is enclosed within a capsule of fibrous tissue, tunica fibrosa. From the deep aspect of the fibrous tunic trabeculae pass inwards to support the glandular tissue. In the superficial part of the cortex the trabeculæ 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 trabecula. These cells contain a lipoid substance, which is present in sufficient quantity to give the cortex a yellow colour.
The medulla is formed of a spongework of cell columns bounding anastomosing venoas 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 in 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 zon. fasciculata. In the zona reticulata the blood-vessels open up to form a venous plexus, which is continued through large sinuses in the medulla to reach a central vein. This is the ve which emerges at the hilus.
Development of the Cortical System and of the Suprarenal Glands.-The cortical system. is a derivative of the cœlomic epithelium (mesoderm). The first indication of its developmen 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 ani penetrate the mesenchyme at the sides of and ventral to the aorta. In an 8-mm. embryo the buds have already lost their connexion with the colomic epithelium. By the time the embry is 9 mm. long the developing cortical masses are vascularised. In man the greater part of the tissue thus formed is ultimately included in the cortex of the suprarenal glands, but small massmay escape, either at this stage or subsequently, to form independent cortical bodies.
In 12-mm. embryos the developing suprarenal glands lie in a caudalward continuation of the dorsal portion of the pleuro-peritoneal membrane called the suprarenal ridge, and are composed cortical tissue only. When the embryo is about 20 mm. in length sympatho-chromaphil 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 masses by sympatho-chromaphil cells has not been observed earlier than the stage mentioned.
At first the iminigrating cells are scattered in numerous columns and strands, and it is 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 long after birth. The zona reticularis develops early and is recognisable in a 15-mm. embryo; th zona glomerulosa is not formed until the second or third year, but is represented until then by layer of small incompletely specialised cells immediately under the fibrous capsule.
During foetal life the cortex is relatively enormous. This is due to a great proliferation ci the cells of the foetal zona reticularis, which differ from the adult cells of the same zone in containing lipoids. This foetal cortex begins to undergo a fatty degeneration soon after birth and by the end of the first year has disappeared. The new cortex which replaces it develop from the small, superficially placed, incompletely specialised cells already referred to.
Comparative Anatomy of the Chromaphil and Cortical Systems. A knowledge of the main facts of the comparative anatomy of the chromaphil system is a help to understanding 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. I 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 re cognisable in the cyclostomata, in which it is arranged in small lobulated masses in the walls the posterior cardinal veins and renal arteries. Even in this rudimentary form it is rich. lipoids. In the rays (elasmobranchs) the cortical system is represented by a pair of yellow coloured, rod-shaped structures in the region of the kidney. In batrachians the chromaphil ar cortical representatives first begin to come together. In the frog the adrenals are golden yel =
ks on the ventral surface of the kidney. The greater part of these are made up of columns ortical cells, but at the borders or ends of the cell columns masses of chromaphil cells occur. sarrangement is transitional between the complete independence of the chromaphil and cortical ems in elasmobranchs and the partial inclusion of chromaphil tissue within cortical charristic of the higher mammals (Swale Vincent). But even in man the union of the systems is from complete. All the chromaphil tissue except the medulla of the suprarenal gland lies of touch with cortical substance. It appears therefore (i.) that the paraganglia of the pathetic trunk are homologous with the segmental chromaphil bodies of elasmobranchs; (ii.) the chromaphil bodies of the abdominal plexuses are a new formation confined to the higher ebrates; (iii) that the inclusion of chromaphil tissue within a capsule of cortical tissue, as he suprarenal glands, is a still later development confined to the highest classes of animals.
2. THE DUCTLESS GLANDS OF ENTODERMAL ORIGIN.
A group of ductless glands, (i.) the thyreoid gland, (ii.) the parathyreoid ands, (iii.) the thymus, and two pairs of inconstant, apparently functionless ructures, (iv.) the cervical thymus glands, and (v.) the ultimo-branchial bodies, e developed from the entodermal lining of the embryonic pharynx.
(i.) GLANDULA THYREOIDEA.
The thyreoid gland (O.T. thyroid body) is placed in the neck, firmly bound y fibrous tissue to the anterior and lateral aspects of the upper part of the rachea and to the sides of he larynx. It is yellowish red, soft, and vascular. It aries in size with age, sex, and general nutrition, being relatively large in youth, in females, and in the well nourished. In women it increases temporarily with menstruation and pregnancy. Its average dimensions are, height 5 cm., breadth 6 cm., weight 25 gm.; but these measurements are of little value because of the range of variation.
Superior thyreoid vein
Internal jugular vein
Isthmus of thyreoid gland
Conventionally, the thyreoid gland is said to consist of two conical lobes united across the middle line by a narrow strand of gland tissue, the isthmus. To many thyreoids this description is inapplicable. In men and thin elderly spinsters the gland is not uncommonly horse-shoe shaped; in young well-nourished women and in pregnancy its general contour suggests a sphere, deeply notched superiorly to accommodate the larynx and deeply grooved posteriorly for the trachea and esophagus. Rarely, the gland is in two parts. Not infrequently, it is asymmetrical. In about 40 per cent of specimens a process of gland tissue, the pyramidal lobe, extends from the upper border of the isthmus, upwards, in front of the cricoid and thyreoid cartilages, towards the hyoid bone. This process is seldom median, lying more often on the left than on the right. In rare cases, it is double. Less rarely, it is double below and single above. Sometimes it is represented by a strip of fibrous tissue or a narrow muscle (lig. suspensorium, or m. levator, glandulæ thyreoidea).
Small oval accessory thyreoid glands are common in the region of the hyoid bone, and are occasionally met with in relation to the right and left lobes.
The relations of the gland are variable, depending upon its size and its relative level.
Common carotid artery
Inferior thyreoid vein
FIG. 1058.-DISSECTION OF THE THYREOID GLAND AND OF THE PARTS
In a majority of cases the isthmus covers the second, third, and fourth rings of the 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 from: 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 moulde. by the sides of the trachea and lower part of the larynx, and, when large, comes int contact, behind them, with the pharynx and oesophagus. 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 ser Parathyreoid Glands, Relations).
Blood and Lymph Vessels.-The blood supply is effected through the superior thyreo arteries, branches of the external carotids, and through the inferior thyreoid arteries, branches of the thyreo-cervical trunks. Occasionally a fifth artery is present, the thyreoidea ima, 1 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 thyreoi 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 gland cr. 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 number Most of these pass to the internal jugulars. A free, transverse, venous anastomosis is effectel 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 th 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 framework for the gland tissue proper. This consists of spheroidal vesicles, 04 mm. to 1 mm. in diameter, lined with cubical epithelium, and filled wit. "colloid." The size, shape, and cellular characters of the vesicles vary with diet and enviro 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 fro the pharyngeal floor (entoderm). It is recognisable as a shallow bay in a 18-mm. embry. practically simultaneously with the demarkation of the foregut. As the bud grows its e 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. T is the tuberculum impar. It migrates forwards, the duct backwards, so that in a 5-mm. embry 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 sebin quite complete, a vestige of the duct, the foramen cæcum of the tongue, remaining in the adult
While these processes are proceeding growth and lateral expansion of the bud continue. It becomes bilobed and has a divided lumen, and all the while it undergoes a continuous relative displacement caudalwards. Soon its lumen disappears. In a 9-mm. embryo the developing thyreoid gland is a transverse bar composed of transversely disposed cell columns. At about ti tenth week of development, 55-mm. embryo, the formation of vesicles commences but is not corpleted until after birth. The remaining changes are due to simple growth and the mouling effects of the pressure of surrounding structures.
The thyreoid gland does not arise in any of its parts from any of the pharyngeal on (see Ultimo-branchial Bodies).
The developmental history of the gland affords a ready explanation of its variations in t` adult. Thus the development of a pyramidal lobe and its variations, partial and compet duplication, are due to the development of gland tissue from that part of the thyreo-glossal which has a double lumen and the more or less complete fusion or separation of the masses !!! formed. Accessory thyreoid glands near the hyoid bone are the result of a similar process connexion with isolated remnants of the duct.
The occurrence in the adult of a duct leading from the foramen cæcum to, or towards, t hyoid bone (lingual duct) is due to a persistence of the upper part of the thyreo-glossal dr Similarly, thyreo-glossal cysts are due to the persistence of short intermediate lengths of the da
(ii) GLANDULE PARATHYREOIDEÆ.
The parathyreoid glands (O.T. parathyroid bodies; Synonyms. epithe bodies, parathymic glands, branchiogenic glands) are finely granular, yellows brown, lenticular or spheroidal structures, from 2 to 20 mm. in diameter and fr
1 to 3 gm. in weight. Most commonly they are lenticular, 5-7 mm. in length, 2 mm. in thickness, and from 01 to 1 gm. in weight. Normally there are two pairs parathyreoids, distinguished by the Roman numerals IV. and III. to signify that ey develop from the fourth and third pharyngeal pouches. Sometimes, in course development, the parathyreoid buds divide so that more than four, five to twelve, ands may be present: the numerals are then applied to the groups of glands rmed from the pouches.
Parathyreoid IV. is commonly embedded in the tunica propria of the thyreoid gland d lies posterior to the corresponding lateral lobe of that organ, about its middle. Parayreoid III., similarly embedded, usually lies on the posterior aspect of the inferior exemity of the lateral lobe of the thyreoid gland. As a rule the anastomosing channel, hich connects the inferior and superior thyreoid arteries (see Thyreoid Gland, Blood apply), passes near both parathyreoids and furnishes the best guide to their discovery, but e range of the exceptional positions which the glands may occupy is wide. Thus, parayreoid IV. may be found (1) behind the pharynx or oesophagus, (2) in the fibrous tissue the side of the larynx, above the level of the thyreoid gland, (3) behind any part of the orresponding lobe of the thyreoid gland or even embedded in the thyreoid substance nternal parathyreoid); whereas parathyreoid III. may be found (1) near the bifurcation the common carotid artery, (2) behind any part of the corresponding lobe of the yreoid gland, (3) on the des of the trachea, or (4) the thorax.
Blood-vessels. - The lood supply of each parahyreoid is effected by a ngle artery which enters he gland at its hilum. It ay spring from any branch f the inferior or superior hyreoid arteries, but most ommonly is a branch of the arge anastomosing channel lready referred to.
Internal jugular vein
Common carotid artery
Structure. The parahyreoids are built up of inercommunicating trabeculæ subclavian vein
of epithelial cells with trands of vascular connecive tissue between them. The cells are of two kinds, one clear, the other, the minority, containing oxyphyl granules. Sometimes they urround spaces recalling Chyreoid vesicles but there s no formation of "colloid," except possibly after thyeoidectomy.
Left lobe of thyreoid gland
Isthmus of thyreoid
Common carotid artery
Development. - The parathyreoid bodies develop from the dorsal diverticula of the third and fourth pharyngeal pouches. The first indication of their development is a proliferation and thickening of the epithelium on the cranial and lateral aspects of the diverticula. This is present in both the third and fourth pouches in 9-10 mm. embryos but appears to be rather irregular in the time of its appearance. The cells forming it are vacuolated, difficult to stain, and indistinct in outline. Cords of cells grow out from the thickening and fibrous tissue penetrates between the outgrowing cords, which soon lose their connexion with the pharynx. The differentiation of the two kinds of cells takes place after birth.
FIG 1059.-THYMUS IN A FULL-TIME FETUS HARDENED BY
is impossible to In various ages.
Parathyreoid III. is normally drawn by the thymus, as it migrates, caudal to parathyreoid IV As a rule it halts at the level of the inferior extremity of the lateral lobe of the thyreoid gland, but may continue its into the remains near the bifurcation of the common carotid artery, where it is apt to be confused with the glandula carotica. It is from this confusion that the idea, that the chromaphil glandulver, or may be q carotica arises from the third pharyngeal pouch, has obtained a foothold in anatomical teaching
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Bifurcation of innominate artery Right subclavian vessels Internal mammary artery
RIGHT LOBE OF
FIG. 1060.-DISSECTION TO SHOW THE THYMUS GLAND IN AN ADULT FEMALE.
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the thymus. This is an irregular pinkish mass of glandular tissue placed in the lower part of the neck and in the superior and anterior mediastina. Its size varies, relatively and absolutely, with age, sex, and nutrition, being relatively largest in infancy, absolutely largest at puberty; larger in females and the well nourished than in males and the emaciated; large in healthy adults accidentally killed; small in persons, even children, who have died of a slow wasting disease. It is supposed that it undergoes a premature permanent involution as the result of severe illness even though, to all appearance, the individual completely recovers.
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