DUCTLESS GLANDS ON THE TRACHEA 215 



The right pulmonary artery is longer than the left, passes 

 to the root of the right lung, behind the ascending aorta and 

 superior vena cava, where it divides into three primary 

 branches, one for each lobe. 



The left pulmonary artery is connected at origin with arch 

 of aorta by ligamentum arteriosum, and passes in front of 

 the descending aorta and left bronchus to the root of the left 

 lung, there dividing into two primary branches for the two 

 lobes. 



DUCTLESS GLANDS ON THE TRACHEA. 



THE THYROID GLAND. 



The thyroid gland is situated on the upper part of the 

 trachea, and consists of two lateral lobes united at lower part 

 by neck or isthmus, which lies over 2nd, 3rd, and 4th rings 

 of trachea and is covered by skin, superficial and deep fasciae, 

 and anterior jugular veins. 



Each lobe is conical, with smaller end upwards. 



Dimensions. Vertical, i inches ; transverse, inch ; thick, 

 inch. 



Relations : 



Antero-external (convex), surface. Deep cervical fascia, sterno- 

 mastoid, sterno - thyroid, sterno - hyoid, and omo-hyoid 

 muscles. 



Postero-internal (concave), surface. Upper six rings of trachea, 

 cricoid and thyroid (below oblique line), crico-thyroid and 

 inferior constrictor muscles, inferior thyroid artery, recurrent 

 laryngeal and external laryngeal nerves, oesophagus (on left 

 side) . 



Postero-external (convex] surface. Sheath of carotid vessels. 



Vessels. Arteries, superior and inferior thyroid ; veins, 

 superior, middle, and inferior thyroid. 



THE THYMUS GLAND. 



The thymus gland attains its full size at two years, after 

 which it gradually shrinks away. It is situated partly in the 

 anterior mediastinum, and partly in the neck, reaching from 

 level of 4th costal cartilage upwards on trachea as high as 



