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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 

Structure.—The thyroid gland is invested by a thin capsule of connective tissue, which projects into its substance and imperfectly divides it into masses of irregular form and size. When the organ is cut into, it is of a brownish-red color, and is seen to be made up of a number of closed vesicles, containing a yellow glairy fluid, and separated from each other by intermediate connective tissue (Fig. 1176).


FIG. 1176– Section of thyroid gland of sheep. X 160. (See enlarged image)

  The vesicles of the thyroid of the adult animal are generally closed spherical sacs; but in some young animals, e. g., young dogs, the vesicles are more or less tubular and branched. This appearance is supposed to be due to the mode of growth of the gland, and merely indicates that an increase in the number of vesicles is taking place. Each vesicle is lined by a single layer of cubical epithelium. There does not appear to be a basement membrane, so that the epithelial cells are in direct contact with the connective-tissue reticulum which supports the acini. The vesicles are of various sizes and shapes, and contain as a normal product a viscid, homogeneous, semifluid, slightly yellowish, colloid material; red corpuscles are found in it in various stages of disintegration and decolorization, the yellow tinge being probably due to the hemoglobin, which is thus set free from the colored corpuscles. The colloid material contains an iodine compound, iodothyrin, and is readily stained by eosin. According to Bensley 1 the thyroid gland prepares and secretes into the vascular channels a substance, formed under normal conditions in the outer pole of the cell and excreted from it directly without passing by the indirect route through the follicular cavity. In addition to this direct mode of secretion there is an indirect mode which consists in the condensation of the secretion into the form of droplets, having high content of solids, and the extension of these droplets into the follicular cavity. These droplets are formed in the same zone of the cell as that in which the primary or direct secretion is formed.
  This internal secretion of the thyroid is supposed to contain a specific hormone which acts as a chemical stimulus to other tissues, increasing their metabolism.

Vessels and Nerves.—The arteries supplying the thyroid gland are the superior and inferior thyroids and sometimes an additional branch (thyroidea ima) from the innominate artery or the arch of the aorta, which ascends upon the front of the trachea. The arteries are remarkable for their large size and frequent anastomoses. The veins form a plexus on the surface of the gland and on the front of the trachea; from this plexus the superior, middle, and inferior thyroid veins arise; the superior and middle end in the internal jugular, the inferior in the innominate vein. The capillary bloodvessels form a dense plexus in the connective tissue around the vesicles, between the epithelium of the vesicles and the endothelium of the lymphatics, which surround a greater or smaller part of the circumference of the vesicle. The lymphatic vessels run in the interlobular connective tissue, not uncommonly surrounding the arteries which they accompany, and communicate with a net-work in the capsule of the gland; they may contain colloid material. They end in the thoracic and right lymphatic trunks. The nerves are derived from the middle and inferior cervical ganglia of the sympathetic.
 
4b. The Parathyroid Glands
 
  The parathyroid glands (Fig. 1177) are small brownish-red bodies, situated as a rule between the posterior borders of the lateral lobes of the thyroid gland and its capsule. They differ from it in structure, being composed of masses of cells arranged in a more or less columnar fashion with numerous intervening capillaries. They measure
Note 1.  American Journal of Anatomy, 1916. xix. [back]

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