THE ORGANS OF RESPIRATION 



239 



Their free ends, which are directed posteriorly, are 

 united similarly and by plain muscular tissue. Its 

 upper limit is at the sixth cervical, its lower, opposite 

 the body or upper border of the fifth thoracic vertebra, 



FIG. 91 



THYROID 

 IMPRESSION 



Trachea and bronchi, front view. (Testut.) 



and it measures about 4J inches in length; transversely, 

 f to 1 inch. Its inner surface is lined by a mucous 

 membrane which belongs to the stratified, ciliated 

 variety of tissues, and this cilia possesses a per- 

 petual movement, carrying the particles of dust, etc., 



