The thyroid is made up of groups of minute closed sacs embedded in a stroma of connective tissue, lined with a single row of epithelium cells, and filled with a clear fluid containing mucin. In the adult the sacs are commonly much distended with a colloid substance and peculiar crystals, and the epithelium has disappeared from their walls. Although said to be rich in lymphatics and to contain follicular tissue, positive proof of the relation of the thyroid body to the lymphatic system is still wanting.

Portion of Thymus removed from its envelope and unraveled so as to show the lobules (b, b) attached to a central band of connective tissue (a).

Fig.158. Portion of Thymus removed from its envelope and unraveled so as to show the lobules (b, b) attached to a central band of connective tissue (a).

Magnified section of a portion of injected Thymus, showing one complete lobule.

Fig. 159. Magnified section of a portion of injected Thymus, showing one complete lobule, with soft central part (cavity) (6), and parts of other lobules. (Cadiat.) (a) Lymphoid tissue, (c) Blood vessels, (d) Fibrous tissue.

Thymus Gland

The functional activity of the thymus is restricted to that period of life when growth takes place most rapidly. It is well developed in the foetus, and increases in size for a couple of years after birth; but it gradually diminishes in bulk and loses its original structure during the later periods of childhood, so as to become completely degenerated and fatty in the adult. It is composed of numerous little follicles of lymphoid tissue collected into groups or lo.bules connected to a kind of central stalk. The lymphoid follicles of the young thymus have some likeness to those of the intestinal tract, but they differ from these agminate glands not only in arrangement but also in having peculiar small nests of large cells (corpuscles of Hassall) in the midst of the adenoid tissue of which they are made up. On account of the structure of the lobules being so nearly identical with that of a lymphatic gland, and from its great richness in lymphatic vessels, the thymus is said to be related to the lymphatic system, and is supposed to play an importent part in the elaboration of the blood during the earlier stages of animal life.

Elements of Thymus (high power). (Cadiat.) (a) Lymph corpuscles, (b) Epithelioid nests of Hassall.

Fig. 160. Elements of Thymus (high power). (Cadiat.) (a) Lymph corpuscles, (b) Epithelioid nests of Hassall.