EPARTERIAL BRONCHIAL SYSTEM. 143 



gration of the cephalic trunk A, or of its secondary branch A\ 

 usually only on the right side, producing apparent asymmetry. 

 This migration affords an opportunity for more complete devel- 

 opment of the resulting terminal bronchial system, and for con- 

 sequent increase in respiratory area. 



5. In the majority of mammals this greater development of 

 respiratory surface is confined to the right side, resulting in the 

 formation of the so-called " eparterial bronchus," and also in- 

 dicated by the development of a special accessory cardiac 

 bronchus of the right side. 



This physiological preponderance of right over left lung is 

 especially well shown by the arrangement of the right lung in 

 artiodactyls (^.^., antelope), where the migration of the cephalic 

 right bronchus has carried the same cephalad, beyond the bi- 

 furcation, to the trachea, and where the resulting voluminous 

 upper lobe of the right lung at times extends completely across 

 the mid-line to cap the apex of the more rudimentaiy left lung. 



6. Except, therefore, for purposes of topography we should 

 abandon the distinction of eparterial and hyparterial bronchi, 

 at least to the extent of clearly recognizing the fact that in 

 asymmetrical lungs every right '' eparterial " bronchus finds its 

 morphological equivalent among the '* hyparterial " bronchi of 

 the left side. 



7. The impropriety of ascribing any morphological signifi- 

 cance to the number of pulmonary lobes is apparent. The di- 

 vision into lobes is an entirely secondary character, not depend- 

 ent upon the type of the bronchial distribution, but probably 

 connected with unequal mobility in different segments of the 

 thoracic walls. Lobe -formation is also subject to a considerable 

 range of variation. 



8. For the reasons above detailed the primitive type of the 

 mammalian lung is the symmetrical " bilateral hyparterial form," 

 the symmetrical " bilateral eparterial form" representing the 

 cnd-stagc in the process of evolution, not the beginning (Aeby, 

 Wiedersheim). 



9. The primitive type of division is practically dichotomous 

 i^Hystrix^ Taxided). 



