THE THYROID APPARATUS 27 



opens at the foramen caecum linguae, but which becomes obliterated 

 during fcetal life. The developed thyroid contains numerous 

 vesicles or follicles, formed by the subdivision of the embryonal 

 tubuli. 



The parenchyma of the thyroid, which is exceedingly vascu- 

 lar, consists of a stroma formed of connective tissue, in the meshes 

 of which the follicles are situated. These follicles are surrounded 

 by a network of capillaries and are bound together by means of 

 connective tissue to form lobules. The follicles are hollow spaces 

 of different sizes and shapes, lined with cubical or cylindrical 

 epithelium. The epithelial cells, in both man and animals, contain 

 a varying number of small and large granules. These are situated 

 in the free end of cells, facing the lumen of the follicle. These 

 granules are highly refractive and, from the reactions which they 

 give, are believed to consist of fat in combination with oleic 

 acid. Except during the first month of fcetal life, these granules 

 are always present; they continuously increase in number until 

 old age is reached ; and it is probable that they play a part in 

 the formation of the secretion. 



The follicle contents are composed of a peculiar substance, 

 known as thyroid 'colloid. Morphologically it is a colloidal sub- 

 stance, though chemically it is not. It varies in consistencyand 

 is insoluble in water, alcohol, ether, and diluted acids. It swells 

 in acetic acid and strong alkalis, and is, in general, acidophile. 

 It stains with acid aniline dyes and most readily with eosin. 

 After staining by van Gieson's method it assumes an orange 

 tone, the connective tissue and hyaline substance becoming a 

 brilliant fuchsine red. As a general rule, it does not stain with 

 the basic aniline dyes, hagmatoxylin, or the specific stains for 

 mucus, though at times it appears to be readily basophile. 

 Nearly all authors are agreed that the colloid contents of the 

 follicles are a product of the secretion of the epithelial cells, though 

 it is probable that a very small proportion is also evolved in the 

 course of cell-decomposition. In sections of thyroid gland pre- 

 pared by the Ehrlich-Biondi method, Langendorff was able to 

 distinguish between principal cells and colloid cells, the former 

 remaining uncoloured, the latter staining red and their nuclei 

 green. The secretory processes of the thyroid have since been 

 closely studied by many authors (Andersson, Bozzi, Galleotto, 

 Hiirthle, L. R. Miiller, E. Schmidt), and it is now generally 

 accepted as a fact that specific granules are formed in the proto- 

 plasm of the epithelial cells, which swell up and pass out of the 

 cell body in the form of drops, and that these drops constitute 

 the contents of the follicles. The various phases of secretory 

 activity are accompanied by characteristic changes in the structure 

 of the cells. The follicular contents, which at first are fluid, 

 become viscid and undergo secondary changes, until finally they 

 attain the characteristic colloid consistency. 



