DUCTLESS GLANDS, in anatomy. A certain number of glands in the body, often of great physiological importance, have no ducts (Lat. ductus, from ducere, to lead, i.e. vessels, tubes or canals for conveying away fluid or other substance); and their products, known as internal secretions, are at once carried away by the veins or lymphatics which drain them. Among these structures are the spleen, the adrenals, the thyroid gland, the parathyroids, the thymus and the carotid and coccygeal bodies. In addition to these the lymphatic glands are described in the article on the lymphatic system (q.v.), and the pineal and pituitary bodies in the article on the brain (q.v.).
From D.J. Cunningham, Cunningham's Text-book of Anatomy.
Fig. 1. - The Spleen - Visceral Aspect.
The human spleen is an oval, flattened gland, of a dull purple colour, and about 5 in. long by 3 broad, situated in the upper and back part of the left side of the abdominal cavity. If the right hand is passed round the left side of its owner's body, as far as it will reach, it approximately covers the spleen. The long axis of the organ is obliquely placed so that the upper pole is much nearer the vertebral column than the lower pole. For practical purposes the long axis of the left tenth rib corresponds with that of the spleen. There is an external or parietal surface and an internal or visceral, the latter of which is again subdivided; these surfaces are limited by ventral and dorsal borders. The external, parietal, or phrenic surface is convex to adapt it to the concavity of the diaphragm, against the posterior part of which it lies; external to the diaphragm is the pleural cavity, and more externally still, the ninth, tenth and eleventh ribs. The internal or visceral surface is divided by a prominent ridge into a gastric or anterior and a renal or posterior surface. Sometimes a triangular impression called the basal surface is formed at the lower part of the visceral surface by the left end of the transverse colon, though at other times no such impression is seen. It is probable that the exact shape of the spleen depends a good deal on the amount of distension of the surrounding hollow viscera at the time of death. (For details of the basal surface see D.J. Cunningham, Journ. Anat. and Phys. vol. xxix. p. 501.) The gastric surface is concave and adapts itself to the fundus of the stomach, while just in front of the ridge separating the gastric and renal surfaces is the hilum, where the vessels enter and leave the organ; in front of this the tail of the pancreas usually touches the spleen. The renal surface is as a rule smaller than the gastric and, like it, is concave; it is moulded on to the upper part of the outer border of the left kidney and just reaches the left adrenal body. The anterior or ventral border of the spleen has usually two or more notches in it, though these are often also seen on the dorsal border. The whole spleen is surrounded by peritoneum, which is reflected off on to the stomach as the gastro-splenic omentum, and on to the kidney as the lieno-renal ligament; occasionally the lesser sac reaches it near its connexion with the pancreas. Small accessory spleens are fairly often found in the neighbourhood of the spleen, though it is possible that some of these may be haemo-lymph glands (see Lymphatic System).
Microscopically the spleen has a fibro-elastic coat in which involuntary muscle is found (fig. 2). This coat sends multitudes of fine trabeculae into the interior of the organ, which subdivide it into numbers of minute compartments, in which the red, highly vascular, spleen pulp is contained. This pulp contains small spherical masses of adenoid tissue, forming the Malpighian corpuscles, situated on the terminal branches of the splenic blood-vessels, together with numerous cells, some of which are red blood corpuscles, others lymph corpuscles, others contain pigment granules or fat, while others have in their interior numerous blood corpuscles. The arteries of the spleen in part end in capillaries from which the veins arise, but more frequently they open into lacunae or blood spaces, which give origin to the veins.
Embryology. - The spleen is developed in the dorsal mesogastrium (see Coelom and Serous Membranes) from the mesenchyme, or that portion of the mesoderm, the cells of which lie scattered in a matrix. Large lymphoid cells are early seen among those of the mesenchyme, but whether these migrate from the coelomic epithelium, or are originally mesenchymal is doubtful, though the former seems more probable. The network of the spleen seems certainly to be derived from cells of the mesenchyme which lose their nuclei.
Fig. 2. - Section of the Spleen seen under a low power.
A, Fibrous capsule.
c, Malpighian corpuscles.
e, Spleen pulp.
Comparative Anatomy. - The spleen is regarded as the remains of a mass of lymphoid tissue which, in a generalized type of vertebrate, stretched all along the alimentary canal. It is absent as a distinct gland in the Acrania and Cyclostomata. In the fishes it is closely applied to the U-shaped stomach, and in some of the Elasmobranchs, e.g. the basking and porbeagle sharks (Selache and Lamna), it is divided into small lobules. In Protopterus among the Dipnoi it is enclosed within the walls of the stomach. In the Anura (frogs and toads) among the Amphibia it is a spherical mass close to the rectum, and this may be explained by regarding it as derived from a different part of the original mass, already mentioned, to that which persists in other vertebrates. In the Iguana among the reptiles the organ has many notches, and each one corresponds to the point of entrance of a vessel. In Mammals the notches, when they are present, so frequently correspond to the points of entrance of arteries at the hilum that the present writer believes that the former are determined by the latter in many cases (see F.G. Parsons on the Notches of the Spleen, J. Anat. and Phys. vol. 35, p. 416; also Charnock Bradley, Proceedings of R. Soc. Edin., vol. 24, pt. 6, p. 521). The Monotremata and Marsupialia have curious Y-shaped spleens. As a rule flesh-eating animals have larger and more notched spleens than vegetable feeders, though among the Cetacea the spleen is relatively very small.
The adrenal glands or suprarenal capsules are two conical bodies, flattened from before backward, resting on the upper poles of the kidneys close to the sides of the vertebral column; each has an anterior and posterior surface and a concave base which is in contact with the kidney. When viewed from in front the right gland is triangular and the left crescentic. On the anterior surface there is a transverse sulcus or hilum from which a large vein emerges. The arteries are less constant in their points of entry, and are derived from three sources, the phrenic, the abdominal aorta and the renal arteries. The glands are entirely retro-peritoneal, though the right one, even on its anterior surface, is very little covered by peritoneum. In a vertical transverse section each gland is seen to consist of two parts, cortical and medullary. The cortical substance is composed of bundles of cells, separated by a stroma, which have a different appearance in different parts. Most superficially is the zona glomerulosa, then the zona fascicularis, and most deeply the zona reticularis. These names convey a fair idea of the appearance of the bundles. To the naked eye the cortical part is yellow while the medullary is red. The medullary part consists of small islets of cells, which resemble columnar epithelium lying among venous sinuses; these cells are said to be in close connexion with the sympathetic nerve filaments from the great solar plexus.
From D.J. Cunningham, Cunningham's Text-book of Anatomy.
Fig. 3. - A, Anterior surface of right suprarenal capsule. B, Anterior surface of left suprarenal capsule. The upper and inner parts of each kidney are indicated in outline. On the right capsule the dotted line indicates the upper limit of the peritoneal covering.
Embryology. - The generally accepted opinion at present is that the cortical substance is derived from the coelomic epithelium covering the mesoderm of the upper (cephalic) portion of the Wolffian body, and corresponds to the nephrostomes of mesonephridial tubules (see Urinary System), while the medullary part grows out from the sympathetic ganglia and so is probably ectodermal in origin. J. Janosik, however (Archiv. f. mikrosk. Anat. bd. xxii. 1883 and Sitzungsber. d. Wiener Akad., 1885), thinks that the cortical part is derived from the germ epithelium covering the upper part of the genital ridge. C.S. Minot (Human Embryology, 1897) believes that the original cells which grow in from the sympathetic disappear later, and that the adult medullary cells are derived from the cortical.
In the early human embryo the adrenals are larger than the kidneys, and at birth they are proportionately much larger than in the adult. (For literature see. Development of the Human Body, J.P. McMurrich, London, 1906; and Handbuch der Entwickelungslehre, by O. Hertwig, Jena.)
Comparative Anatomy. - Adrenals are unknown in Amphioxus and the Dipnoi (mud fish). In the Cyclostomata (hags and lampreys) they are said by some to arise in connexion with the cephalic part of the pronephros, though other writers deny their presence at all (see W.E. Collinge and Swale Vincent, Anat. Anz. bd. xii., 1896). In the Elasmobranchs and Holocephali the medullary and cortical parts are apparently distinct, the former being represented by a series of organs situated close to the intercostal arteries, while the latter may be either median or paired, and, as they are placed between the kidneys, are often spoken of as interrenals. In the Amphibia the glands are sunk into the surface of the kidney. In reptiles and birds they are long lobulated bodies lying close to the testis or ovary and receiving an adrenal portal vein. In the lower mammals they are not as closely connected with the kidneys as they are in man, and their shape is usually oval or spherical.
The Thyroid Gland
The thyroid body or gland is a deep red glandular mass consisting of two lobes which lie one on each side of the upper part of the trachea and lower part of the larynx; these are joined across the middle line by the isthmus which lies in front of the second and third rings of the trachea. Occasionally, from the top of the isthmus, a nearly but not quite median pyramidal lobe runs up toward the hyoid bone, while in other cases the isthmus may be absent. The gland is relatively larger in women and children than in the adult male. It is enclosed in a capsule of cervical fascia and is supplied by the superior and inferior thyroid arteries on each side, though occasionally a median thyroidea ima artery is present. On microscopical examination the gland shows a large number of closed tubular alveoli, lined by columnar epithelial cells, unsupported by a basement membrane, and filled with colloid or jelly-like material. These are supported by fibrous septa growing in from the true capsule, which is distinct from the capsule of cervical fascia. The lymphatic vessels are large and numerous, and have been shown by E.C. Baber (Phil. Trans., 1881) to contain the same colloid material as the alveoli. Accessory thyroids, close to the main gland, are often found.
Embryology. - The median part of the gland is developed from a tube which grows down in the middle line from the junction of the buccal and pharyngeal parts of the tongue (q.v.), between the first and second branchial arches. This tube is called the thyro-glossal duct and is entodermal in origin. The development of the hyoid bone obliterates the middle part of the duct, leaving its upper part as the foramen caecum of the tongue, while its lower part bifurcates, and so the asymmetrical arrangement of the pyramidal lobe is accounted for. A. Kanthack (J. Anat. and Phys. vol. xxv., 1891) has denied the existence of this duct, but on slender grounds. The lateral parts of the gland are developed from the entoderm of the fourth visceral clefts, and, joining the median part, lose their pharyngeal connexion. Nearly, but not quite, the whole of the lateral lobes probably belong to this part. (For literature and further details see Quain's Anatomy, London, 1892, and J.P. McMurrich's Development of the Human Body, London, 1906.)
Comparative Anatomy. - The endostyle or hypobranchial groove of Tunicata (sea squirts) and Acrania (Amphioxus) is regarded as the first appearance of the median thyroid; this is a median entodermal groove in the floor of the pharynx, secreting a glairy fluid in which food particles become entangled and so pass into the intestine. In the larval lamprey (Ammocoetes) among the Cyclostomata the connexion with the pharynx is present, but in the adult lamprey (Petromyzon), as in all adult vertebrates, this connexion is lost. In the Elasmobranchs the single median thyroid lies close to the mandibular symphysis, but in the bony fish (Teleostei) it is paired. In the mud fish (Dipnoi) there is also an indication of a division into two lobes. In the Amphibia the thyroid forms numerous vesicles close to the anterior end of the pericardium. In Reptilia it lies close to the trachea, and in the Chelonia and Crocodilia is paired. In birds it is also paired and lies near the origin of the carotid arteries. In Mammalia the lateral lobes make their first appearance. In the lower orders of this class the isthmus is often absent. (For further details and literature see R. Wiedersheim's Vergleichende Anatomie der Wirbeltiere, Jena, 1902, and also for literature, Quain's Anatomy, London, 1896.)
These little oval bodies, of considerable physiological importance, are two in number on each side. From their position they are spoken of as postero-superior and antero-inferior; the postero-superior are embedded in the thyroid at the level of the lower border of the cricoid cartilage, while the antero-inferior may be embedded in the lower edge of the lateral lobes of the thyroid or may be found a little distance below in relation to the inferior thyroid veins. They are often very difficult to find, but it is easiest to do so in a perfectly fresh, full-term foetus or young child. Microscopically they consist of solid masses of epithelioid cells with numerous blood-vessels between, while, embedded in their periphery, are often found masses of thymic tissue including the concentric corpuscles of Hassall. They have been regarded as undeveloped portions of thyroid tissue in an embryonic state, but the experiments of Gley (Comptes rendus de la Soc. de Biol. No. 11, 1895) and of W. Edmunds (Proc. Physiol. Soc. - Journ. Phys. vol. xviii., 1895) do not confirm this. They are developed from the entoderm of the third and fourth branchial grooves.
Parathyroids have been found in the orders of Primates, Cheiroptera, Carnivora, Ungulata and Rodentia among the Mammalia, and also in Birds. In the other classes of vertebrates little is known of them. The fullest and most recent account of these bodies is that of D.A. Welsh in Journ. Anat. and Phys. vol. 32, 1898, pp. 292 and 380.
The Thymus Gland
The thymus gland is a light pink gland, consisting of two unequal lobes, which lies in the superior and anterior mediastina of the thorax in front of the pericardium and great vessels; it also extends up into the root of the neck to within a short distance of the thyroid gland. It continues to grow until the second year of life, after which it remains stationary until puberty, when it usually degenerates rapidly. The writer has seen it perfectly well developed in a man between 40 and 50, though such cases are rare; probably, however, some patches of its tissue remain all through life. Each lobe is divided into a large number of lobules divided by areolar tissue, and each of these, under the microscope, is seen to consist of a cortical and medullary part. The cortex is composed of lymphoid tissue and resembles the structure of a lymphatic gland (see Lymphatic System); it is imperfectly divided into a number of follicles. In the medulla the lymphoid cells are fewer, and nests of epithelial cells are found, called the concentric corpuscles of Hassall. The vascular supply is derived from all the vessels in the neighbourhood, the lymphatics are very large and numerous, but the nerves, which come from the sympathetic and vagus, are few and small. H. Watney (Phil. Trans., 1882) has discovered haemoglobin, and apparently developing red blood corpuscles, in the thymus. (For further details see Gray's or Quain's Anatomy.)
Embryology. - The thymus is formed from a diverticulum, on each side, from the entoderm lining the third branchial groove, but the connexion with the pharynx is soon lost. The lymphoid cells and concentric corpuscles are probably the derivatives of the original cells lining the diverticulum.
Comparative Anatomy. - The thymus is always a paired gland. In most fishes it rises from the dorsal part of all five branchial clefts; in Lepidosiren (Dipnoi), from all except the first; in Urodela from 3rd, 4th and 5th, and in Anura from the 2nd only (see T.H. Bryce, "Development of Thymus in Lepidosiren," Journ. Anat. and Phys. vol. 40, p. 91). In all fishes, including the Dipnoi (mud fish) it is placed dorsally to the gill arches on each side. In the Amphibia it is found close to the articulation of the mandible. In the Reptilia it is situated by the side of the carotid artery; but in young crocodiles it is lobulated and extends all along the neck, as it does in birds, lying close to the side of the oesophagus. In Mammals the Marsupials are remarkable for having a well-developed cervical as well as thoracic thymus (J. Symington, J. Anat. and Phys. vol. 32, p. 278). In some of the lower mammals the gland does not disappear as early as it does in man. The thymus of the calf is popularly known as "the chest sweetbread."
These are two small bodies situated, one on each side, between the origins of the external and internal carotid arteries. Microscopically they are divided into nodules or cell balls by connective tissue, and these closely resemble the structure of the parathyroids, but are without any thymic tissue. The blood-vessels in their interior are extremely large and numerous. The modern view of their development is that they are part of the sympathetic system, and the reaction of their cells to chromium salts bears this out. (See Kohn, Archiv f. mikr. Anat. lxx., 1907.)
In the Anura there is a rete or network into which the carotid artery breaks up in the position of the carotid body, and this has an important effect on the course of the circulation. It is probable, however, that this structure has nothing to do with the carotid body of Mammalia.
This is a small median body, about the size of a pea, situated in front of the apex of the coccyx and between the insertions of the levatores ani muscles. It resembles the carotid body in its microscopical structure, but is not so vascular. Concentric corpuscles, like those of the thymus, have been recorded in it. It derives its arteries from the middle sacral and its nerves from the sympathetic. Of its embryology and comparative anatomy little is known, though J.W. Thomson Walker has recently shown that numerous, outlying, minute masses of the same structure lie along the course of the middle sacral artery (Archiv f. mikroscop. Anat. Bd. lxiv.). The probability is that, like the carotid body, it is sympathetic in origin. (Quain's Anatomy gives excellent illustrations of the histology of this as well as of all the other ductless glands.)
For the literature on and further details concerning the foregoing structures the following works should be consulted: Quain's Anatomy, vol. 1 (1908, London, Longman & Co.); McMurrich's Development of the Human Body (London, Rebman, 1906); Wiedersheim's Vergleich. Anat. der Wirbeltiere (Jena, 1898).
(F. G. P.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)