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.”
Carotid Bodies
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.
Coccygeal Body
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.)
DUDERSTADT, a town of Germany, in the Prussian province of Hanover, situated in a beautiful and fertile valley (formerly called Goldene Mark) watered by the Hahle, and on the railway Wulften-Leinefelde. Pop. (1905) 5327. It is an interesting medieval town with many ancient buildings. Notable are the two Roman Catholic churches, beautiful Gothic edifices of the 14th century, the Protestant church, and the handsome town-hall. Its chief industries are woollen and cotton manufactures, sugar-refining and cigar-making; it has also a trade in singing-birds. Duderstadt was founded by Henry I. (the Fowler) in 929, passed later to the monastery of Quedlinburg, and then to Brunswick. It was a member of the Hanseatic League, and during the Thirty Years’ War became a stronghold of the Imperialists. It was taken by Duke William of Weimar in 1632; in 1761 its walls were dismantled, and, after being alternately Prussian and Hanoverian, it passed finally in 1866 with Hanover to Prussia.