458 THE BE8PIBAT0BY APPARATUS IN MAMMALIA. 



left, the arch of the aorta and the thoracic duct. The latter is sometimes 

 carried to the opposite side. . 



Steuoturb.— The trachea comprises in its structure : the cartilaginous 

 rings which form its base ; the ligaments which unite these nngs ; the 

 mucous membrane spread over its inner face ; a muscular layer, which only 

 lines that membrane superiorly ; and vessels and nerves. 



Fig. 230. 



THE EESPIKATOET ORGANS; INFERIOR, OR FRONT VIEW. 

 1, Trachea ; 2, Jugular vein ; 3, Great rectus anticus muscle ; 4. Carotid artery ; 

 5,-Longus colli muscle; 6, Origin of the common carotids; 7, Vertebral artery; 

 8, Section of first rib ; 9, Cephalic trunk of right axillary artery ; 10, Anterior 

 lobe of right lung; 11, Middle, or supplementary lobe of ditto; 12, Posterior 

 portion or lobe of ditto ; 13, Heart ; 14, Cardiac artery ; 15, Ventricular branch 

 of cardiac vein ; 16, (Esophagus. 



Cartilaginous rings of the trachea. — These are about fifty in number, 

 and do not form perfect rings, being incomplete on the upper side of the 

 trachea. Each is a kind of arc, composed of a cartilaginous plate flattened 

 and curved on itself, whose extremities are turned towards each other, and 



