236 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



and third cartilage ring. The lateral lobes are closely 

 applied to the sides of the trachea and extend up- 

 ward to cover the lower part of the thyroid cartilage. 

 Each lobe measures about two inches in length, one 

 and one-quarter inches in breadth, and one-half inch 

 in depth. The gland, however, is subject to great 

 variations both in size, form, and position. It is gen- 

 erally larger in females, and appears to undergo a 

 periodic enlargement during each menstruation. It 



reaches its maxi- 

 mum growth at pu- 

 berty, and is fre- 

 quently much di- 

 minished in size in 

 old age. 



Structure. The 

 thyroid gland is in- 

 vested by a thin 

 layer of areolar tis- 

 sue which not only 

 binds it fast to the 

 trachea, but divides 

 it into small lobules 

 of irregular size and 

 form. It is a duct- 

 less gland consisting of epithelial vesicles varying in 

 size from .05 mm. to i mm. in diameter. These vesi- 

 cles vary greatly in form and are grouped and held to- 

 gether by areolar connective tissue in which many 

 blood-vessels ramify. These vesicles are generally 

 filled with colloid substance, a yellow viscid fluid, that 

 in sections stains a yellowish red with eosin. They are 



Fig. 182. Portion of a cross sec- 

 tion of thyroid gland of a man; c, col- 

 loid substance (Sobotta). 



