Thyroid Gland

Thyroid Gland, a gland which arises in the early human embryo as an ingrowth from the lower part of the pharynx. This extends down to the lower part of the neck, and its original connection with the pharynx is lost. In the adult it is found as a bilobed structure on either side of the windpipe, and joined in front of this tube by an isthmus of gland-tissue. It is ensheathed in connective tissue, which passes into its interior, sup- porting and separating from one another the delicate structures within. The gland is a ductless gland, but is richly supplied by blood-vessels, which ramify around the tiny glandular capsules of which it is composed. If a section of the gland be examined with the microscope, it is seen that there is a vast number of tiny spherical cavities lined by short cubical cells; and within and entirely filling these cavities, there is a structureless-looking and very insoluble jelly. In the walls of the little cavities the dense Anastomosis (q.v.) of blood-vessels is seen. We are very much in the dark as to the meaning of this structure, and the parts played by the epithelial cells and the jelly in the function of the gland. That the gland is important is proved by the injurious effects which follow its removal, and by the fact that in connection with several diseases the gland is swollen and altered in structure. In some animals, the dog for instance, the jelly within the capsules is often found to contain red blood-corpuscles. Sometimes large numbers of these are seen, and in other cases the blood pigment (hæmoglobin) may be seen beautifully crystallised out in the jelly. That the gland has something to do with the blood seems very evident, but what part it exactly plays is not yet certain. If the gland be excised, in some animals, curious mucous degenerations are found to follow the operation, and a tendency to mucous deposits and tumours seems to be associated with disease of the gland. Some have thought that this points to some action of the gland in the formation of mucous within the body, but about this little is known. If the gland is excised nervous symptoms and muscular tremors may be produced, and it has been supposed that in some way the gland exercises an influence upon the blood-flow through the brain. The gland is frequently enlarged, forming the condition termed Goitre (q.v.). This enlargement may be due to a great increase in the number of blood-vessels, or we may find that it is due to the enlargement of the little capsules which normally contain the thyroid jelly. These capsules run together so as often to form large cysts full of jelly. Associated with this enlargement one often finds nervous symptoms and disturbances of the circulatory system, and the curious condition termed cretinism. In exophthalmic goitre we have a peculiar protrusion of the eyes associated often with an unimportant-looking swelling in the neck. See also CRETINISM, MYXEDEMA.