70 The Development of the Lungs 
and arteries or the bronchi and veins are injected, or else, in triple in- 
jections where all three systems are filled with different masses. Prepa- 
rations with the artery and veins filled with one color and the bronchi 
another, are relatively easy to obtain, but the more instructive triple 
injections are extremely difficult to make. The changes gradually taking 
place with the growth of the tree, may be followed step by step in these 
cleared and corroded specimens, but they need not be described in detail 
until they are more exaggerated, as shown, for example, in triple corro- 
sions of a pig 15 cm. long. Owing to my inability to find an artist who 
could draw these complicated structures, the reader may perhaps find it 
convenient to follow the following descriptions by means of the metal 
corrosions shown in PI. IV, Figs. 23, 24. The common pulmonary artery 
now divides to the left of the trachea a short distance after its origin from 
the pulmonary arch. The branch to the tracheal bronchus is given off 
from the right pulmonary artery at the left margin of the trachea and, 
after crossing ventralwards to it. divides with Lateral 1 into a dorso- 
inferior and a ventrosuperior branch. The latter passes ventralwards 
to the tracheal bronchus, and, at its point of division, mounts up over 
the ventrosuperior branch and comes to occupy a position dorsal, slightly 
medial, to this bronchus. The dorsoinferior branch passes beneath, and 
runs dorsal to the dorsoinferior bronchus. The right pulmonary artery 
then passes downwards in front of the trachea, and turns back and out 
to oceupy a dorsolateral position to the axial bronchus. Just above the 
second lateral bronchus, the branch to that division of the tree is given 
off, which courses a little above and behind the bronchus sending rami- 
fications to accompany its side bronchi. The dorsoinferior branch crosses 
behind the main bronchus, and runs dorsal to the branch which it 
supplies, leaving that structure between it and the corresponding vein. 
In the remainder of its course, the second lateral branch lies dorsal to 
the bronchus with the bronchus between it and its accompanying vein. 
The branch to Ventral 2 originates just below Lateral 2 and, passing 
underneath its root, winds around the axial bronchus to gain the lower 
and lateral aspect of the Bronchus infracardiacus, which it accompanies 
in its ramification. The dorsal branch to Dorsal 2 runs on the lateral 
surface of the bronchus and is given off from the right pulmonary artery 
near the origin of the bronchus. The third lateral branch hes dorsal- 
wards and sligthly superior to Lateral 3, and ramifies with its branches. 
The branch to the third ventral bronchus arises in a manner similar to 
that of the second, and winds underneath the third lateral bronchus 
around the stem to the lateral aspect of Ventral 3. The artery corre- 
sponding to Dorsal 3 has a similar distribution to the one above. The 
