THYROID GLAND
The thyroid gland bears much resemblance in structure to other glandular organs, and is usually classified together with the thymus, supra-renal glands, and spleen, under the head of ductless glands, from its possessing no excretory duct. Its function is unknown, but, from its situation in connection with the trachea and larynx, is usually described with thfce, although taking no part in the function of respiration. It is situated at the upper part of the trachea, and consists of two lateral lobes, placed one on each side of this tube, connected together by a narrow transverse portion, the isthmus.
Its anterior surface is convex, and covered by the Sterno-hyoid, Sterno-thyroid, and Omo-hyoid muscles.
Its lateral surfaces, also convex, lie in contact with the sheath of the common carotid artery.
Its posterior surface is concave, and embraces the trachea and larynx. The posterior borders of the gland extend as far back as the lower part of the pharynx.
This gland is of a brownish red colour. Its weight varies from one to two ounces. It is larger in females than in males, and becomes slightly increased in size during menstruation. It occasionally becomes enormously hypertrophied, constituting the disease called bronchocele, or goitre. Each lobe is somewhat conical in shape, about two inches in length, and three-quarters of an inch in breadth, the right lobe being rather the larger of the two.
The isthmus connects the lower third of the two lateral lobes: it measures about half an inch in breadth, and the same in depth, and usually covers the second and third rings of the trachea. Its situation presents, however, many variations, a point of some importance in the operation of tracheotomy. Sometimes the isthmus is altogether wanting.
A third lobe, of conical shape, called the pyramid, occasionally arises from the left side of the upper part of the isthmus, or from the left lobe, and ascends as high as the hyoid bone. It is occasionally quite detached, or divided into two parts, or altogether wanting.
A few muscular bands are occasionally found attached, above, to the body of the hyoid bone, and connected, below, with the isthmus of the gland, or its pyramidal process, and named by Soemmering, the Levator glandulae thyroidae.
structure. The thyroid consists of numerous minute closed vesicles, composed of a homogeneous membrane, enclosed in a dense capillary plexus, and connected together into imperfect lobules by areolar tissue. These vesicles are spherical or oblong, perfectly distinct, and contain a yellowish fluid, in which are found floating numerous ' dotted corpuscles' and cells. The fluid coagulates by heat or alcohol, but preserves its transparency. In the foetus, and in young subjects, the corpuscules lie in a single layer, in contact with the inner surface of these cavities, and become detached during the process of growth.
The Arteries supplying the thyroid, are the superior and inferior thyroid, and sometimes an additional branch from the arteria innominata, which ascends from this vessel 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 which arise the superior, middle, and inferior thyroid veins; the two former terminating in the internal jugular, the latter in the vena innominata.
The Lymphatics are numerous, of large size, and terminate in the thoracic and right lymphatic ducts.
The Nerves are derived from the pneumogastric, and from the middle and inferior cervical ganglia of the sympathetic.
Chemical Composition. The thyroid gland consists of albumen, traces of gelatine, stearine, oleine, extractive matter, alkaline, and earthy salts, and water. The salts are chloride of sodium, alkaline sulphate, phosphate of potash, lime, magnesia, and a trace of oxide of iron.
