THYROID GLAND. 145 



The alveoli are round, oval, or polygonal on section (Fig. 

 103), and, taken as a whole, are somewhat bent or coiled closed 

 tubes. They are lined with one layer of cubical or low cylin- 

 drical epithelial cells. These have a nucleus near the centre of 

 the cell, and often refractive granules in the protoplasm. 

 According to some authors, there is a membrana propria out- 

 side the epithelium. The cells formerly known as chief cells 

 and colloid cells represent different stages in secretion seen in 

 the same cell. 



The alveoli contain a colloid substance secreted by the epi- 

 thelial cells. This does not usually fill the whole lumen, but is 



FIG. 103. 





C . S ; o ' '. * . 



H Colloid substance 



Part of a section through a human thyroid. X 180. 



separated from the walls, especially in hardened specimens, by 

 clear spaces, as though the colloid had clung to the walls at 

 various places, but was pulled away in the intervals (Fig. 103). 

 The colloid takes a characteristic eosin stain and appears to be 

 quite homogeneous. Whether this colloid has any part in the 

 formation of the internal secretion which is characteristic of 

 the thyroid is not known. It is possible that the colloid corre- 

 sponds with the external secretion of glands that have ducts, 

 10 



