IHYREOLD GLAND 



227 



age; usually the cells are low columnar or cuboidal. The mature 

 thyreoid gland consists, therefore, of rounded, closed spaces, or follicles, 

 bounded by a simple entodermal epithelium (Fig. 216). The follicles 

 vary greatly in diameter. Generally they are rounded, but sometimes 

 they are elongated, and occasionally they branch or communicate with one 

 another. Among them are cords or clumps of cells which have not 

 acquired a lumen. 



Within the follicles, and forming the most conspicuous feature of the 

 thyreoid gland in ordinary sections, is a hyaline material which stains 



Flat epithelium. 



Blood 

 Connective tissue. vessels. 



Artery with 

 two thickenings. 



Colloid with drops 

 of mucus. 



Oblique section of a follicle. 



FIG. 216. SECTION OF A LOBCLE OF THE THYREOID GLAND FROM AN ADULT MAN. X 220. 



deeply with cosine and is named 'colloid.' The hyaline material in the 

 thymic corpuscles, the hypophysis, and in the coagulum in the cervical 

 blood and lymphatic vessels, has also been designated colloid. In sections 

 of the thyreoid gland it usually does not fill the follicle but has contracted, 

 producing a spiny border. Granules, vacuoles and droplets of mucus, de- 

 tached cells, leucocytes, and crystalloid bodies may be found in it. It is a 

 product of the epithelial cells, in the protoplasm of which similar material 

 has been detected. It has been said that it is transferred to the blood and 

 lymphatic vessels, passing out between the epithelial cells. 



As has been learned by experiment, the thyreoid gland produces an 

 internal secretion which is essential for the normal growth and development 

 of the body. It is, however, not known whether this secretion leaves the 

 basal or free surface of the thyreoid epithelium, and its relation to the 



