THE TRACHEA 913 



The superior laryngeal communicates with this })ranch by a slender filament, 

 which passes downwards near to the posterior border of the thyroid cartilage. 

 This communication sometimes takes place beneatli the posterior crico-arytencnd 

 muscle. 



The arteries are derived from the superior and inferior thyroid, the e})iglottis 

 receiving some twigs from tlie dorsalis lingute of tlic lingual. 



Tlie veins correspond with the arteries; and the lymphatics, which are scanty, 

 follow the vessels and end in the deep cervical glands. 



THE TRACHEA 



The trachea, or air-tube, which is cylindrical in shape, but flattened posteriorly, 

 extends from the lower border of the fifth cervical vertebra to the fourth or fifth 

 thoracic vertebra. It is continuous with the larynx above, and divides into the two 

 bronchi below. It measures from four and a half to five inches (10 to 12 cm.) in 

 length, and is nearly an inch ('I'o cm.) in width. 



Relations. — In its cervical portion it rests upon the oesophagus, wliicli curves 

 somewhat to the left at the root of the neck; on each side, but especially on the 

 left, it comes into relationshij) with the lateral lobes of the thyroid gland, the 

 inferior tliyroid arteries, and the recurrent laryngeal nerves (these latter running 

 upwards gain the lateral groove between the trachea and oesophagus) ; and lastly, 

 it is in relation with the sheath containing the comuKju carotid artery, internal 

 jugular vein, and pneumogastric nerve. 



In front it is crossed, opposite the second, third, and fourth rings, by the 

 isthmus of the thyroid body; above the isthmus it is concealed in part by the 

 pyramidal lobe of the thyroid and by the levator glandule thyroidea muscle, and 

 it is crossed liy the internal terminal branches of the superior thyroid arteries. 

 Below the isthmus the inferior thyroid veins, the thyroidea ima artery, if it is 

 present, and the remains of the thymus gland lie in close relation with it, and 

 more .anteriorly, separated from it by the deep cervical fascia, are the anterior 

 jugular veins and their anastomosis above the sternum. 



It is overlapped on each side l)y the sterno-hyoid, sterno-thyroid, and sterno- 

 niastoid muscles, and the interval between the muscles of the two sides is crossed 

 by a strong deep and a thinner superficial layer of deep ceiwical fascia, whilst still 

 more superficially lie the superficial fascia and skin. 



In its thoracic portion the trachea still rests U)ton and retains its comiection 

 with the (esophagus, wliicli separates it from the sjiine. It lies between the two 

 |)leural sacs and pneumogastric nerves.. In front of it is tlie sternum with the 

 origins of the sterno-hyoid and sterno-thyroid muscles, the thymus gland, the deep 

 cardiac plexus, the aortic arch which crosses the tube just above its bifurca- 

 tion, the connnencements of the innominate and left connnon carotid arteries and 

 the left innominate vein which crosses the roots of the latter vessels. On its right 

 sifle are the pleural sac, the vagus nerve, and the innominate artery, and on its left 

 side the left connnon carotid and subclavian arteries, the left vagus and left recur- 

 rent nerves, and the left pleural sac. 



Structure. — The trachea is made up of a series of imperfect cartilaginous riniis. 

 deficient behind, connected throughout by fibro-elastic mend)rane, and behind with 

 muscular fibres (the trachealis muscle), a special layer of yellow elastic fibres, and 

 a lining of mucous memitranc. 



The cartilaginous rings vary in number from sixteen to twenty. They are 

 incomplete, being deficient in the hinder third, and are connected in a continuous 

 series by a fibrous membrane, which divides to enclose them, but reunites in the 

 narrow intervals l)etween. It forms a definite layer where the cartilages are want- 

 ing, so that it mav be regarded as complete throughout the tube. The inside of 

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