712 ORGANS OF VOICE AND RESPIRATION. 



flattened posteriorly, extending from the lower part of the larynx, on a level with 

 the fifth cervical vertebra, to opposite the third dorsal, where it divides into the 

 two bronchi, one for each lung. The trachea measures about four inches and a 

 half in length ; its diameter, from side to side, is from three-quarters of an inch to 

 an inch, being always greater in the male than in the female. 



Relations. The anterior surface of the trachea is convex, and covered, in the 

 neck, from above downwards, by the isthmus of the thyroid gland, the inferior 

 thyroid veins, the arteria thyroidea ima (when that vessel exists), the Sterno- 

 hyoid and Sterno-thyroid muscles, the cervical fascia (in the interval between 

 these muscles), and, more superficially, by the anastomosing branches between the 

 anterior jugular veins; in the thorax, it is covered from before backwards by the 

 first piece of the sternum, the remains of the thymus gland, the arch of the aorta, 

 the innominate and left carotid arteries, and the deep cardiac plexus. It lies 

 upon the oesophagus, which is directed to the left, near the arch of the aorta ; 

 laterally, in the neck, it is in relation with the common carotid arteries, the lateral 

 lobes of the thyroid gland, the inferior thyroid arteries, and recurrent laryngeal 

 nerves ; and, in the thorax, it lies in the interspace between the pleuras, having the 

 pneumogastric nerve on each side of it. 



The Right Bronchus, wider, shorter, and more horizontal in direction than the 

 left, is about an inch in length, and enters the right lung, opposite the fourth dorsal 

 vertebra. The vena azygos arches over it, from behind ; and the right pulmonary 

 artery lies below, and then in front of it. 



The Left Bronchus is smaller, more oblique, and longer than the right, being 

 nearly two inches in length. It enters the root of the left lung, opposite the fifth 

 dorsal vertebra, about an inch lower than the right bfonchus. It crosses in front 

 of the oesophagus, the thoracic duct, and the descending aorta ; passes beneath the 

 arch of the aorta, and has the left pulmonary artery lying at first above, and then 

 in front of it. If a transverse section is made across the trachea, a short distance 

 above its point of bifurcation, and a bird's-eye view taken of its interior (fig. 362), 



the septum placed at the bottom of 



Fie. 362. Transverse Section of the Trachea, ,-, , \ ,. ,-, > > 



just above its bifurcation, with a bird's-eye tlj . ls tube > separating the two bronchi, 



view of the interior. will be seen to occupy the left of the 



median line, as was first shown by Mr. 

 Goodall, of Dublin, so that any solid 

 body descending the trachea, by virtue 

 of the laws of gravity, would naturally 

 be directed towards the right bronchus, 

 and this tendency is undoubtedly aided 

 "by the larger size of this tube, as 

 compared with its fellow. This fact 

 serves to explain why a foreign sub- 

 stance in the trachea almost universally falls into the right bronchus. 



The trachea is composed of imperfect cartilaginous rings, fibrous membrane, 

 muscular fibres, longitudinal yellow elastic fibres, mucous membrane, and glands. 

 The Cartilages vary from sixteen to twenty in number ; each forms an imper- 

 fect ring, which surrounds about two-thirds of the cylinder of the trachea, being 

 imperfect behind, where the tube is completed by fibrous membrane. The carti- 

 lages are placed horizontally above each other, separated by narrow membranous 

 intervals. They measure about two lines in depth, and half a line in thickness. 

 Their outer surfaces are flattened, but, internally, they are convex, from being 

 thicker in the middle than at the margins. The cartilages are connected together, 

 at their margins, by an elastic fibrous membrane, which covers both their surfaces ; 

 and in the space between their extremities, behind, forms a distinct layer. The 

 peculiar cartilages are the first and the last. 



The first cartilage is broader than the rest, and sometimes divided at one end ; 

 it is connected by fibrous membrane with the lower border of the cricoid cartilage, 

 with which, or with the succeeding cartilage, it is sometimes blended. 



