778 ORGANS OF VOICE AND RESPIRATION. 



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 

 those 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 recur- 

 rent 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 bronchus. 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 



Fig. 429. Transverse Section of the Trachea, transverse section is made across the 

 just above its Bifurcation, with a bird's eye trachea, a short distance above its point 



of bifurcation, and a bird's eye view 

 taken of its interior (Fig. 429), the sep- 

 tum placed at the bottom of the trachea 

 and separating the two bronchi 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 dropping 

 into the trachea, would naturally be di- 

 rected 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 substance in the trachea generally 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 im- 

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

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

 cartilages are placed horizontally above each other, separated by narrow mem- 

 branous 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 con- 

 nected 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 car- 

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

 The last cartilage is thick and broad in the middle, in consequence of its lower 



