1226 



THE RESPIRATORY SYSTEM 



Position and relations (figs. 972, 989, 1000). — The trachea lies in the median 

 plane, extending from the level of the sixth cervical vertebra downward and 

 backward, receding from the surface in following the curve of the vertebral col- 

 umn, and deviating a little to the right in approaching the level of the fourth 

 thoracic vertebra, where it divides. Its lower end is fixed so that with elevation 

 and descent of the larynx the tube is stretched and contracted, changes in length 

 which also result from extension and fiexion of the head and neck. The mobility 

 of the trachea is favored by its loose investment of connective tissue. 



About half of the trachea lies in the neck, but the extent varies with the length of the neck, 

 the position of the head and with age; the trachea holds a lower position in adult life than in 

 childhood and a still lower one in old age when the bifurcation may be as low as the sixth or 

 seventh thoracic vertebra. In front and closely connected with it is the isthmus of the thyreoid 

 gland, covering usually the second to fourth cartilages; anterior to this the cervical fascia and 

 integuments. The cervical aponeurosis is attached to the upper margin of the sternum in two 

 lameUse, with an interspace containing the venous jugular arch, a lymph gland, and some fat. 

 Between these aponeuroses and the trachea is another space containing the inferior thyreoid 

 veins and some tracheal lymph-glands, and sometimes a thyreoidea ima artery. The innominate 

 artery occasionally crosses the trachea obhquely in the root of the neck. Behind the trachea, 

 in its whole length, Ues the oesophagus, which in this part of its course inclines to the left. 

 On either side are the great vessels and nerves of the neck, and the lobes of the thyreoid gland. 

 The inferior laryngeal nerve Ues in the angle between the oesophagus and trachea. 



Fig. 990. — Bifurcation of the Trachea showing the Tracheal Keel. R. L. Right and 

 left bronchi. (HeUer and von Schroetter, from Poirier and Charpy.) 



Within the thorax the trachea lies in the mediastinum, enveloped in loose areolar tissue 

 and fixed through strong fibrous connections with the central tendon of the diaphragm. The 

 innominate artery and the left common carotid are at first in front and then at its sides as they 

 ascend, while the left innominate vein and the remains of the thymus are further forward. 

 The aortic arch is in contact with the anterior surface of the trachea near the bifurcation. 

 On the right side are the vagus nerve, the arch of the vena azygos, the superior vena cava, and 

 the mediastinal pleura; on the left, the arch of the aorta, the left subclavian artery, and the 

 recurrent laryngeal nerve. A large group of bronchial lymph-glands [lymphoglandulae bron- 

 chiales] lies below the angle of bifurcation. The oesophagus is behind and to the left. 



The bronchi take an obUque course to the hilus of the lung, where they branch. 

 The right bronchus is nearer to the vertical in its course than is the left; it is 

 also shorter and broader. These conditions, together with the position of the 

 tracheal keel, explain the more frequent entrance of foreign bodies into the right 

 than into the left bronchus. The asymmetrical course of the two bronchi is 

 probably genetically associated with the position of the heart and aorta. 



The azygos vein arches over the right bronchus, the vagus passes behind, and the right 

 branch of the pulmonary artery crosses anteriorly below the level of the first (eparterial) 

 branch of the bronchus. The aorta arches over the left bronchus and gains its posterior surface 

 along with the oesophagus; the left branch of the pulmonary artery passes at first in front and 

 then above the bronchus. 



Dimensions. — On account of their elasticity considerable difficulty is met with in obtaining 

 accurate measurements of the air-tubes. The length of the trachea is given at 95-122 mm.; 

 its transverse diameter 20-27 mm.; the .sagittal diameter 16-20 mm. The right bronchus has 

 a length of 25-34 mm.; the left, 41-47 mm. The transverse diameter of the right is 18 mm.; of 

 the left, 16 mm. The angle of bifurcation of the trachea varies from 56° to 90°, the mean 

 being 70.4° a wide angle corresponding tf) the breadth of the thorax of man. The right bronchus 

 makes an angle of 24.8° with the median plane; the left, 45.6°. 



According to Tillaux the Icngtli of tliat portion of the trachea between the superior edge of 

 the sternum and the cricoid cartilage varies with age and sex as follows: — 



Adult male, from 4.5 to 8.5 cm average, 6.5 cm. 



Adult female, " 5 to 7.5 cm " 6.4 cm. 



Boys 2i to 10 years, " 2.7 to 6.5 cm " 4.4 cm. 



Girls 3i to lOi " " 4 to 6.5 cm " 5.1cm. 



