EPARTERIAL BRONCHIAL SYSTEM. 137 



as the lateral division of B'\ appear as lateral (secondary) 

 branches derived from the parent-stem. We have the dichot- 

 omous type of division of the primitive form replaced by the 

 monopodic origin of lateral branches from a main parent or 

 stem-bronchus, which condition characterizes the lung of the 

 higher mammalia. 



b. Rig] it Side. 



The first fact noticed is the complete separation of the branches 

 A and A' and the consequent elimination of the primary cephalic 

 trunk A. A' has migrated slightly dorsad and cephalad, so as 

 to arise from the stem-bronchus near the bifurcation. A" has 

 shifted ventrad and slightly caudad on the stem-bronchus. The 

 interval thus opened between them by the elimination of the 

 trunk A is utilized by the right pulmonary artery to gain the 

 dorso-lateral aspect of the stem-bronchus. 



In general there can be no question as to the morphological 

 equivalence, regarding direction, size and lung area supplied, of 

 the branches A' and A" on right and left sides. The same is 

 true regarding the corresponding branches of the pulmonary 

 artery. To be noted is the early derivation of the arterial 

 trunk accompanying A' on the right side ; also the somewhat 

 more pronounced independent character oi A' ^ revealed by the. 

 greater number and size of its lateral secondary and tertiary de- 

 rivatives, all facts accentuating the physiological importance 

 which the apical portion of the right lung has assumed. 



The caudal segment follows in the main the type presented 

 by the left side. We recognize the same character and deriva- 

 tion of the stem-bronchus. 



A new element, not represented on the left side, appears as 

 the cardiac bronchus ((T), derived from the stem-bronchus 

 (segment B) caudad and mesad to the separate origin of A" . 

 Comparison with the bronchial tree of Taxidca shows that the 

 large artery, accompanying the cardiac bronchus and supplying 

 the Azygos lobe, corresponds topographically to the arterial 

 branch which in Taxidca is seen to course ventro-mesad be- 

 tween A and A' and B to reach the bronchi derived from B' . 



The cardiac bronchus appears as a secondary structure im- 



