THE THORAX. 357 



from collapsing like the oesophagus by the introduction of 

 incomplete rings of cartilage. 



These cartilaginous arches are incorporated within the 

 structure of the tube. Their opening is placed behind, and 

 the gap filled in with muscular tissue. The trachea is the 

 continuation of the air tube below the larynx, or the cricoid 

 cartilage. It terminates at its bifurcation into the two 

 bronchi at the fourth or fifth thoracic vertebra. Its length 

 is about five inches and its diameter nearly an inch. 



Relations. In the neck : In front, skin, fasciae, anterior 

 jugular veins, their communicating and the inferior thyroid 

 veins, the isthmus of the thyroid gland, branches of the 

 superior thyroid artery. Laterally (and in front), sterno- 

 hyoid, sternothyroid muscles, lobes of the thyroid gland. 

 Laterally, lobes of the thyroid gland carotid sheath and its 

 contents. Behind, oesophagus (and at sides, recurrent 

 laryngeal nerves and the inferior thyroid arteries). 



In the chest : In front, origin of muscles as above. Re- 

 mains of the thymus gland. Left innominate vein, trans- 

 verse aorta, and the diverging innominate and left common 

 carotid arteries, the deep cardiac plexus. Laterally, the 

 lungs and their pleurae, the pneumogastric nerves, and on 

 the left side the recurrent laryngeal nerve. Behind, the 

 oesophagus. 



The blood supply is from the inferior thyroid arteries, the 

 nerve supply from the pneumogastrics, recurrent laryngeals, 

 and sympathetic nerves. 



The Bronchi. Fig. 73. Diag. 22. 



These are constructed the same as the trachea, of which 

 they are the terminal branches. 



The bifurcation of the trachea is not at the middle plane 

 of the trachea, but more to the left side, consequently the 



