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THE THYMUS
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THE THYMUS GLAND
The thymus gland (Gr. O os, from a fancied resemblance to the corymhs of the See also:Thyme) is a See also:light See also:pink gland, consistingof two unequal lobes, which lies in the See also:superior and anterior mediastina of the, See also:thorax in front of the pericardium and See also:great vessels; it also extends up into the See also:root of the See also:neck to within a See also:short distance of the See also:thyroid gland. It continues to grow until the second See also:year of See also:life, after which it remains stationary until See also:puberty, when it usually degenerates rapidly. The writer has seen it perfectly well See also:developed in a See also:man between 40 and 50, though such cases are rare; probably, however, some patches of its See also:tissue remain all through life. Each See also:lobe is divided into a large number of lobules divided by areolar tissue, and each of these, under the See also:microscope, is seen to consist of a cortical and medullary See also:part. The cortex is composed of lymphoid tissue and resembles the structure of a lymphatic gland (see LYMPHATIC See also:SYSTEM); it is imperfectly divided into a number of follicles. In the medulla the lymphoid cells are fewer, and nests of See also:epithelial cells are found, called the concentric corpuscles of Hassall. The vascular See also:supply is derived from all the vessels in the See also:neighbour-See also:hood, 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 See also:blood corpuscles, in the thymus. (For further details see See also: The lymphoid cells and concentric corpuscles are probably the derivatives of the See also:original cells lining the diverticulum. See also: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. See also:Bryce, " Development of Thymus in Lepidosiren," Journ. Anat. and Phys. vol. 40, p. 91). In all fishes, including the Dipnoi (mud See also:fish) it is placed dorsally to the gill See also:arches on each side. In the See also:Amphibia it is found See also:close to the See also:articulation of the mandible. In the Reptilia it is situated by the side of the See also:carotid artery; but in See also:young crocodiles it is lobulated and extends all along the neck, as it does in birds, lying close to the side of the See also: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 See also:lower mammals the gland does not disappear as See also:early as it does in man. Additional information and CommentsThere are no comments yet for this article.
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