Page:The American Cyclopædia (1879) Volume XV.djvu/763

 THYROID GLAND TIARA 733 tinum, extending in childhood from the thyroid gland to the anterior surface of the pericardium, but becoming atrophied after the age of -pu- berty. It is divided into two lateral portions, right and left, which are in reality distinct from each other, being connected only by are- olar tissue. Each lateral portion is gland-like in structure, being divided into a number of lobules, from a sixth to a third of an inch in diameter, and irregularly rounded and flattened. In its interior is a central cavity, having the form in some cases, according to Kolliker, of a cylindrical canal, communicating with that of the separate lobules, and containing a gray- ish white or milky-looking fluid, of a slightly acid reaction. Each lobule is invested on its exterior by a thin, nearly homogeneous mem- brane, which sends partitions a short distance into its substance, dividing it in this way ex- ternally into rounded masses or gland folli- cles, each about 7 ^ of an inch in diameter, giving the outer surface of the lobule a gran- ular appearance. The solid substance of the lobule consists of a soft homogeneous material, containing nucleated cells and an abundance of free nuclei, and penetrated throughout by capillary blood vessels, which radiate from the wall of the central cavity and terminate in vas- cular loops toward its external portion. Its central cavity has no excretory duct, and its secreted product, if such there be, must be taken up and carried away by the veins or the lymphatics. The thymus gland is highly de- veloped during the latter part of intra-uterine life, and at the time of birth, in man, weighs rather more than half an ounce. It continues to enlarge until the age of two years, at which time its growth ceases. It begins to diminish about the 10th year, but is still usually per- ceptible, and sometimes well developed, at the age of 20. By the 40th year it has entirely disappeared. It is about the same in the an- thropoid apes as in man, and is remarkably developed and may be well studied in the calf, in which, and in the lamb, it is called the sweetbread, and is a delicate article of food; it exists in mammals, birds, and most reptiles, but not in the larvae of batrachians, the peren- nibranchiate amphibians, or fishes. The pre- cise function of the thymus gland is unknown. It undoubtedly serves, like the other ductless glands, to accomplish some change in the blood circulating through its tissue, which is essential to the proper nourishment of the body during intra-uterine life, infancy, and childhood. THYROID GLAND (Gr. 6vpe6<;, a shield, and etffoc, form), one of the vascular or ductless glands, situated on the anterior and lower part of the larynx, in front of the upper rings of the trachea ; so called from its being situated in front of the thyroid or shield-like cartilage of the larynx. It is composed of two elon- gated ovoid lobes, flattened from before back- ward, united or separate, but generally con- nected by a transverse portion ; it is covered by the muscles of the front of the neck. The tissue is tolerably firm, brownish and yellow- ish red, formed of lobes and lobules, consist- ing essentially of an aggregation of closed gland follicles imbedded in condensed areolar tissue ; these contain a small amount of a fatty albuminous fluid, and do not communicate with any common reservoir ; among the follicles are nucleated corpuscles or epithelial cells ; it re- ceives four arteries from the subclavians and the carotids, nerves from the pneumogastric and the sympathetic, and lymphatics communi- cating with the glands of the neck. The vas- cular supply is great, and forms a very minute capillary plexus on the membrane of the folli- cles; like other ductless glands (see THYMUS GLAND), it is relatively larger in intra-uterine existence and in infancy than in after life. Its products are probably discharged into the ve- nous blood, and serve for the elaboration of the circulating fluid. It is usually larger in fe- males than in males ; it is found in all mam- mals, birds, and reptiles, probably in the batra- chians, and perhaps also in fishes. The organ may be inflamed, with suppuration, and vari- ously enlarged. In the adult it is sometimes ab- normally enlarged, forming the disease known as goitre or bronchocele, which is itself often an accompaniment of cretinism. (See GOITKE.) TIAGUANACO, or Tiahuanaco. See TITICAOA. TIARA (Gr.), a species of high hat anciently worn by many eastern nations. Those of kings and priests were encircled by a sort of crown, whence the term tiara has been applied to the triple crown worn by the popes, which in turn FIG. 1. Persian Tiara. Fie. 2. Assyrian Tiara. bears a striking resemblance to the Persian royal tiaras and to those of the Assyrian kings, represented on the slabs at Nineveh. It is un- certain when the popes assumed the tiara. It was called regnum, or emblem of royalty, to distinguish it from the mitre. Pope Innocent III., in his sermon on St. Syl- vester, says that the bishop of Rome uses the mitre every- where and at all times, because his episcopal jurisdiction is universal, whereas he only uses the regnum or tiara occasion- ally and in his own dominions. The papal tiara was at first a conical cap, topped with a small round ball, and wreathed about the forehead with a crown of gold. It retained this shape