

THE TEACHEA. 



1079 



These differences in the calibre of the tube are determined by the surroundings of 

 the trachea. The cervical part is narrowed owing to its being clasped by the thyreoid 

 gland. Further, a short' distance above its bifurcation, an impression, sometimes 

 strongly marked, is usually seen on the left side of the trachea. It is due to the 

 close contact of the aortic arch as it passes dorsally against that part of the tube. 

 It is evident therefore that the second slight diminution in calibre which is described 

 by Braune and Stahel is pro- 

 duced by the proximity of the 

 aorta. Lejars gives the average 

 ventro-dorsal diameter of the 



trachea in the living person as & U'^mm^r Thyreoid cartilage 



11 mm., and the transverse 

 diameter as 12'5 mm. In the 

 dead subject the lumen of the 

 tube is considerably greater. 



viciue 



right 

 ward 



ico-thyreoid membrane 



Cricoid cartilage 



I Part of trachea covered by 



isthmus of thyreoid gland 



Common carotid artery 



Eparterial bronchus 

 Hyparterial bronchus 



The trachea adheres 

 rigorously to the median 

 plane except towards its 

 termination, where it de- 

 viates very slightly to the 



ht. As it passes down- 

 .8 it recedes rapidly from 

 the surface. This is due to 

 its following the curvature 

 of the vertebral column, from 

 which it is separated by the 

 cesophagus alone. 



Relations of the 

 Trachea. In the study of 

 the relations of the trachea 

 it is convenient to consider 

 it in the two stages of cer- 

 vical and thoracic. 



When the chin is held 

 so that the face looks directly 

 forwards the cervical part of 

 the trachea measures from 

 2 to 2 1 inches in length ; but 

 when the head is thrown 

 dorsally or backwards the 

 length is considerably in- 

 creased. It is clasped by the 

 thyreoid gland, the isthmus 

 of which is applied to its 

 ventral surface, and covers 

 the second, third, and fourth 

 rings ; while on each side 

 the corresponding lobe of the 

 thyreoid gland is applied 

 to the side of the trachea 

 and extends downwards to 

 the fifth or sixth ring. On 

 either side of the cervical part of the trachea is the common carotid artery, whilst 

 the recurrent nerve passes upwards in the groove between the trachea and the 

 oesophagus. Dorsally the trachea is in relation to the oesophagus, which intervenes 

 between it and the bodies of the vertebrae and deviates somewhat to the left as it 

 passes downwards. 



In addition to the isthmus of the thyreoid gland two thin muscular strata, 

 composed of the sterno-hyoid and the ster no- thyreoid muscles, as well as the deep 



69 & 



Pulmonary artery 



FIG. 858. THE TRACHEA AND BRONCHI. 

 The thyreoid gland is indicated by a dotted line and a purple tint. 



