. The chordates. Chordata. Integrative Systems 221. ''JJ*A'i _ CONNECTIVE VX//'T ISSUE i $% BLOOD VESSEL FOLLI CLE .CU BO I DAL EPITHELIUM Fig. 188. A portion of a section of the thyroid gland, enlarged, showing the secretory epithelium and the colloid-filled follicles. (Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) lies ventral to the trachea and just posterior to the larynx. It is usually a single median body but in some cases, especially among amphibians and mammals, it becomes more or less definitely bilobed. The thyroid is formed of numerous
. The chordates. Chordata. Integrative Systems 221. ''JJ*A'i _ CONNECTIVE VX//'T ISSUE i $% BLOOD VESSEL FOLLI CLE .CU BO I DAL EPITHELIUM Fig. 188. A portion of a section of the thyroid gland, enlarged, showing the secretory epithelium and the colloid-filled follicles. (Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) lies ventral to the trachea and just posterior to the larynx. It is usually a single median body but in some cases, especially among amphibians and mammals, it becomes more or less definitely bilobed. The thyroid is formed of numerous masses of glandular tissue, separated from one another by connective tissue (Fig, 188). The glan- dular tissue consists of rounded follicles, each enclosed by a single layer of cuboidal epithelium. Loose connective tissue filled with blood- vessels and lymphatics binds the follicles together. Each follicle is filled with a jelly-like colloid material which has a strong affinity for acid dyes such as eosin. The most important, if not the exclusive, endocrinal secretion of the thyroid gland is an iodine-containing compound, thyroxin, whose empiric chemical formula is C15H11O4NI4. A substance with identical properties has been made synthetically, and it is generally the synthetic drug which is used in medical practice. One function of thyroxin is to control metabolism, especially that of the carbohydrates. Also, it regulates growth before birth, through infancy, and at puberty. Tadpoles fed with thyroid may become frogs prematurely, but of abnormally small size. Excessive activity of the human gland is a common malady, manifested by extreme nervous- ness, rapid pulse, insomnia, and basal metabolism above normal, so that the patient loses weight. As too much thyroxin speeds up the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble th
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