ining ridge running sagitally, or, as the
patient lies in the recumbent position, we speak of it as being
vertical. On either side are seen the openings of the right and left
main bronchi. In Fig. 44, it will be seen that the lower border of the
carina is on a level with the upper portion of the orifice of the
right superior-lobe bronchus; with the carina as a landmark and by
displacing with the bronchoscope the lateral wall of the right main
bronchus, a second, smaller, vertical spur appears, and a view of the
orifice of the right upper-lobe bronchus is obtained, though a lumen
image cannot be presented. On passing down the right stem bronchus
(patient recumbent) a horizontal partition or spur is found with the
lumen of the middle-lobe bronchus extending toward the ventral surface
of the body. All below this opening of the right middle-lobe bronchus
constitutes the lower-lobe bronchus and its branches.

[FIG. 45.--Bronchoscopic views.
S; Superior lobe bronchus; SL, superior lobe bronchus; I, inferior
lobe bronchus; M, middle lobe bronchus.]

[56] Coming back to the carina and passing down the left bronchus, the
relatively great distance from the carina to the upper-lobe bronchus
is noted. The spur dividing the orifices of the left upper- and
lower-lobe bronchi is oblique in direction, and it is possible to see
more of the lumen of the left upper-lobe bronchus than of its
homologue on the right. Below this are seen the lower-lobe bronchus
and its divisions (Fig. 45).

_Dimensions of the Trachea and Bronchi_.--It will be noted that the
bronchi divide monopodially, not dichotomously. While the lumina of
the individual bronchi diminish as the bronchi divide, the sum of the
areas shows a progressive increase in total tubular area of
cross-section. Thus, the sum of the areas of cross-section of the two
main bronchi, right and left, is greater than the area of cross
section of the trachea. This follows the well known dynamic law. The
relative increase in surface as the tubes branch and diminish in size
increases the friction of the passing air, so that an actual increase
in area of cross section is necessary, to avoid increasing resistance
to the passage of air.

The cadaveric dimensions of the tracheobronchial tree may be
epitomized approximately as follows:
Adult
Male Female Child Infant
Diameter trachea, 14 X 2