. The endocrine organs; an introduction to the study of internal secretion . FIG. 29.—Medulla of thymus of child. Magnified 300 diameters. The smalldarkly stained cells are lymphocytes. The preparation shows two or threeconcentric corpuscles, and blood-vessels. and a medulla which is lighter (rig. 28). The difference is due to the largenumber of lymphoid cells in the cortex, which is densely packed with them. The medulla is formed ofa cell-reticulum as a basis, in themeshes of which are a considerablenumber of lymphoid cells; and inaddition the characteristic concen-tric corpuscles of Hassal,
. The endocrine organs; an introduction to the study of internal secretion . FIG. 29.—Medulla of thymus of child. Magnified 300 diameters. The smalldarkly stained cells are lymphocytes. The preparation shows two or threeconcentric corpuscles, and blood-vessels. and a medulla which is lighter (rig. 28). The difference is due to the largenumber of lymphoid cells in the cortex, which is densely packed with them. The medulla is formed ofa cell-reticulum as a basis, in themeshes of which are a considerablenumber of lymphoid cells; and inaddition the characteristic concen-tric corpuscles of Hassal, whichare formed by nests of epitheliumcells surrounding one or more cellswhich occupy the centre of the nest(figs. 29, 30). Some of the concen-tric corpuscles are compound, havingmore than one centre. Occasionallythe medulla is found to containsmall ciliated cavities, and rarelystriated muscle-fibres are seen with-. FIG. 30.—A concentric corpuscle of the thymuswith a part of the adjoining reticulum.(Hammar.) c., a ciliated vesicle. in it. The thymus is developed in theembryo from sprouts of the epithelium of the third branchial pouch oneach side (fig. 2). These unite to form a single median column of cellswhich becomes hollowed out and tubular, and ultimately branched. Even-tually lymphoid nodules forming the cortex of the lobules appear in the Functions of the Thymus 47 walls of this branching tube (tig. 31), and its epithelium becomes broken up and modified to form the reticulum and concentric corpuscles of the medulla. In Petromyzon the thymus retains its original connexion with the branchial clefts, but in all other vertebrates this connexion becomes lost, and in mammals the organ takes up a position either in the neck or in the anterior mediastinum. In birds it forms a series of isolated masses on each side of the neck. Small portions of thymus tissue are frequently found embedded in the t
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Keywords: ., bo, bookcentury1900, bookdecade1910, bookidendocrineorgansi00shar
