458 



THE RESPIRATORY APPARATUS IN MAMMALIA. 



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

 carried to the opposite side. 



STRUCTURE. The trachea comprises in its structure : the cartilaginous 

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

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

 lines that membrane superiorly ; and vessels and nerves. 



Fig. 230. 



THE RESPIRATORY 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 



