ORGANS OF RESPIRATION. 



235 



Vein. 



covered with a very thin mucous membrane. They 

 are attached anteriorly to the thyroid cartilage, 

 close to each other, and diverge posteriorly to their 

 attachment in the arytenoid cartilages. The glottis 

 is the slit-like opening between the vocal cords. 



THE THYROID GLAND. 



The thyroid gland is not a part of the respiratory 

 tract, but it is so closely associated with this tract in 

 development and in position that it seems advisable 

 to describe the 

 organ in this 

 place. The gland 

 is a highly vas- 

 cular body con- 

 sisting of tw r o lat- 

 eral lobes and 

 connected by a 

 transverse bar, 

 the isthmus . The 

 rudiments of the 

 lobes develop 

 from the epithe- 

 lium of the fourth gill cleft, while the isthmus and a 

 large part of the lobes come from the floor of the 

 mouth, the thyroglossus duct at the base of the 

 tongue being a remnant of this origin. The gland 

 is therefore an epithelial organ derived from the ento- 

 derm. 



In position the gland lies low down in the neck 

 and in close apposition to the trachea. The isthmus 

 crosses in front of the trachea and covers the second 



Artery. 



Colloid. 



Connective tissue. 



Fig. 181. Section from thyroid gland, show- 

 ing vesicles or alveoli. 



