. Principles of modern biology. Biology. The Endocrine Glands - 403 below the larynx (Fig. 22-3). The entire thy- normal. Conversely, an excess of thyroxin roid weighs only about an ounce; and the elevates the metabolic rate considerably above normal levels. Many of the severe symptoms that ac- company thyroid derangement probably rep- gland is purely endocrine in function. -LARYNX ALVEOLI CONTAINING COLLOID. TRACHEA GROSS VIEW Fig. 22-3. Thyroid gland: structure and position. MICROSCOPIC VIEW The thyroid arises in all vertebrate em- bryos as an outgrowth from the floor of the pharynx. All ver

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. Principles of modern biology. Biology. The Endocrine Glands - 403 below the larynx (Fig. 22-3). The entire thy- normal. Conversely, an excess of thyroxin roid weighs only about an ounce; and the elevates the metabolic rate considerably above normal levels. Many of the severe symptoms that ac- company thyroid derangement probably rep- gland is purely endocrine in function. -LARYNX ALVEOLI CONTAINING COLLOID. TRACHEA GROSS VIEW Fig. 22-3. Thyroid gland: structure and position. MICROSCOPIC VIEW The thyroid arises in all vertebrate em- bryos as an outgrowth from the floor of the pharynx. All ver Stock Photo
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. Principles of modern biology. Biology. The Endocrine Glands - 403 below the larynx (Fig. 22-3). The entire thy- normal. Conversely, an excess of thyroxin roid weighs only about an ounce; and the elevates the metabolic rate considerably above normal levels. Many of the severe symptoms that ac- company thyroid derangement probably rep- gland is purely endocrine in function. -LARYNX ALVEOLI CONTAINING COLLOID. TRACHEA GROSS VIEW Fig. 22-3. Thyroid gland: structure and position. MICROSCOPIC VIEW The thyroid arises in all vertebrate em- bryos as an outgrowth from the floor of the pharynx. All ver
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. Principles of modern biology. Biology. The Endocrine Glands - 403 below the larynx (Fig. 22-3). The entire thy- normal. Conversely, an excess of thyroxin roid weighs only about an ounce; and the elevates the metabolic rate considerably above normal levels. Many of the severe symptoms that ac- company thyroid derangement probably rep- gland is purely endocrine in function. -LARYNX ALVEOLI CONTAINING COLLOID. TRACHEA GROSS VIEW Fig. 22-3. Thyroid gland: structure and position. MICROSCOPIC VIEW The thyroid arises in all vertebrate em- bryos as an outgrowth from the floor of the pharynx. All vertebrates also possess para- thyroid glands (p. 405), which arise from the wall of the pharynx. In many species the parathyroids lie some distance from the thy- roid, but in man the parathyroids are four small bean-shaped bodies, embedded in the thyroid tissue. The importance of the thyroid was sus- pected even in ancient times; but in 1885 replacement experiments gave definite proof of a thyroid hormone; and in 1916 thyroxin (C15Hu04NI4) was isolated for the first time. Thyroxin successfully counteracts the effects of thyroidectomy, although the hormone ap- pears to be carried in the blood stream as a thyroxin-protein compound.:i The primary effect of the thyroid hormone is upon the rate of metabolism in the body tissues. With inadequate thyroxin, the body consumes less oxygen and produces less meta- bolic wastes; and in extreme cases the basal metabolism falls to less than 30 percent of 3 Recently a series of compounds, closely related to thyroxin, have been isolated from thyroid tissues; among these, tri-iodothyronine (T3) is about five times more active than thyroxin. resent secondary effects, resulting from the drastic metabolic changes in the various or- gans. Also the net results are different, de- pending upon whether the thyroid deficiency (or excess) is experienced before or after the animal reaches maturity. Hypothyroidism in young animals retards development very pro

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