198 



NORMAL HISTOLOGY. 



meshes are filled with vascular mesodermic tissue; (c) severance of the 

 epithelial reticulum into masses corresponding to the later follicles; (d) the 

 appearance within these masses of lumina, around which the cells become 

 arranged as the epithelial lining of the compartments subsequently containing 

 the characteristic colloid substance. The thyroid body agrees with the 

 parathyroids and the thymus in arising from the walls of the primitive 

 pharynx and in deviating during its later development from its original 

 likeness to a typical gland. 



The thyroid body is situated in the neck, in front and at the sides of 

 the upper end of the trachea, and consists of two lateral lobes connected by 

 a narrow strip, the isthmus. Although during its early development cor- 



Interlobular 

 septum 



Connec- 

 tive tissue 



FIG. 246. Section of thyroid body, showing follicles in various degrees of distention. X 100. 



responding in principle with compound alveolar glands, the fully formed 

 thyroid body possesses no excretory ducts and varies in the details of its 

 terminal compartments. The fibro-elastic capsule investing the organ gives 

 off septa which subdivide the lobes into a number of tracts, each composed 

 of smaller masses, the primary lobules, separated by thin partitions of con- 

 nective tissue. These subdivisions (.5-1 mm. in diameter) contain a variable 

 but usually large number of follicles, which correspond to the alveoli of 

 ordinary glands and are supported by a highly vascular fibro-elastic frame- 

 work. 



The follicles, ellipsoidal or cylindrical sacs, vary greatly in size (50- 

 2co,), depending upon the amount of the contained secretion and disten- 

 tion. They are lined by a single layer of fairly regular epithelial cells, 

 usually cuboidal, although they may approach the columnar or flattened 

 type. Their spherical nuclei are surrounded by clear cytoplasm, which 

 often contains granules of a fatty nature. The epithelial cells are the source 

 oi tne peculiar soft acidophil substance, colloid, the material that fills and 

 distends to a variable degree the follicles. Some of the latter may appear 

 very small and tubular and contain no secretion, while the neighboring fol- 

 licles are enormously distended with masses of colloid. As usually seen in 

 sections, the colloid substance is homogeneous or finely granular and often 

 partly detached from the lining cells by shrinkage. Vacuoles are also 



