THE TRACHEA. 923 
cartilage, and opposite the lower margin of the sixth cervical vertebra. From this 
it extends downwards through the lower part of the neck into the superior media- 
stinum of the thorax, in which it ends at the level of the upper border of the fifth 
dorsal vertebra by dividing into the right and left bronchus. The leneth of the 
trachea in the male is from four to four rn a half inches, and in the female from 
three and a half to four inches, but even in the same individual it varies consider- 
ably in length with the movements of the head and neck. 
The lower end of the trachea is fixed in position. This is a necessary provision to 
prevent dragging on the roots of the lungs during movements of the head and neck. 
The remainder of the tube is surrounded by a quantity of loose areolar tissue, and 
possesses a considerable amount of mobility. Further, its wall is highly elastic, and thus 
when the head is thrown back the tube elongates through stretching, and when the chin 
is depressed its length is 
diminished by the recoil of its 
wall. 
ae Thyroid cartilage 
The trachea does not r 
present an absolutely uni- 
form calibre throughout its \ 
whole length. About its 
middle it exhibits a slight 
expansion or dilatation, and 
from this the calibre dimin- 
ishes in an upward and a : 
downward direction. Close 
to the bifurcation it is again x 
shghtly expanded (Braune % 
and Stahel). 
These differences in the 
calibre of the tube are in some 
measure determined by the 
surroundings of the trachea. 
The upper part is apparently 
narrowed through its being 
clasped by the thyroid body. «tt . ae 
Further, a short distance above 
the bifurcation an eer 
sometimes strongly marked, 
frequently seen on the lett 
side of the trachea. This is 
due to the close contact of the 
aortic arch as it passes back- 
wards against this part of the 
tube. It is therefore probable 
that the second slight diminu- 
tion in calibre which is de- 
scribed by Braune and Stahel 
is produced by the proximity 
of the aorta. Lejars gives the 
average antero-posterior dia- 
meter of the trachea in the 
living person as 11 mm., and 
the transverse diameter as 
125mm. In the dead subject 
the lumen of the tube is con- 
siderably greater. 
—Crico-thyroid membrane 
Cricoid cartilage 
Part of trachea covered by 
‘ . . . . 
| isthmus of thyroid body 
Common carotid artery 
Subelavian artery 
Left bronchus 
Yi 
Pulmonary artery 
Hyparterial 
bronehi 
parterial bronchus 
Hyparterial bronchus 
é Pulmonary artery 
he aes Fia. 625.—THE TRACHEA AND BRONCHI. 
The tracheaadheresrigor- 2 bkales Ar ene 
7: : The thyroid body is indicated by a dotted line. 
ously to the median line 
except towards its lower end, where it deviates very slightly to the right. As it 
descends 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. 
i 
