PULMONARY EVOLUTION IN MAMMALIA 177 
relations of the left stembronchus cranial to the origin of V! to 
the foregut, vagus nerve and pulmonary artery are less favorable 
for this type of bronchial development than on the right side, 
for the reasons referred to below (cf. infra, p. 186). In place of 
this the ascending branch (A) of V! has unfolded more fully, 
until in many cases it nearly or entirely equals in size and extent 
Ep. of the right side. Like the latter it supplies the cranial 
lung area of its side. The ventro-lateral extension of V! is often 
considerably reduced as compared with the homologous bronchus 
of the right lung. This is largely due to the projection of the 
heart into the left thoracic cavity, necessitating a diminution in 
the volume of the ventro-caudal portion of the left upper lobe. 
This is also indicated by the frequent cardiac recession of the 
left ventral pulmonary margin. As the result of these conditions 
the districts of the left lung supplied by A and V! respectively are 
in many forms not separated by a secondary interlobar incisure, 
such as divides on the right side the ‘upper’ from the ‘middle’ 
lobe. Both remain included within the limits of the primitive 
left ‘upper’ lobe. That this can, however, undergo the same 
lobar subdivision as in the right lung, is shown by numerous 
mammalian specific types. 
With the full development of the conditions just described 
the mammalian lung has reached the evolutionary stage attained 
by over 90 per cent of the living forms. It enters the dominant 
group, forming Aeby’s type II*, with the eparterial bronchus 
derived only from the right stembronchus. Extension of the 
same evolutionary process to the left lung produces Aeby’s type 
I, the symmetrical bilateral eparterial tree of bronchial deriva- 
tion. If a point on the tracheal epithelium is selected for pro- 
liferation of the right eparterial component the typical artio- 
dactyl and cetacean lung results (Aeby’s type II®), and if in 
addition the left stembronchus cranial to V! develops an epar- 
terial bronchus, the lung of Camelidae and of Giraffa is formed 
(Aeby’s type 1°). 
The origin of the eparterial bronchus may be so low that barely 
enough space is left between it and V! for the passage of the pul- 
monary artery. On the other hand it may arise from the most 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 2 
