Apeil 15, 1898.] 



SCIENCE, 



521 



mammalia whicli were investigated, a dis- 

 tinct and progressive series can be estab- 

 lished in accordance with which the various 

 conditions of bronchial distribution and 

 vascular supply may be derived from the 

 primitive type found in Hystrix and Taxidea. 

 This series is illustrated in the paper by a 

 selection of the following seven forms : Ca- 

 nis familiaris, Dicotyles torquatus, Myrmeco- 

 phaga juhata, Auchenia glama-pacos, Cebus ca- 

 pueinus, Cebus niger, Phoca vitulina. 



Aeby's ' stem bronchus ' and its mono- 

 podic system of lateral branches, character- 

 istic of the majority of mammalian lungs, 

 appears to be derived from the tracheal 

 bulla or lacuna with dichotomous division 

 of the primary branches found in Hystrix 

 and Taxidea by further development and 

 relative rearrangement. The typical ' stem 

 bronchus ' develops from three segments of 

 the primitive bronchial tree. 



1. Proximal portion, between the bifur- 

 cation and the origin of the primary ce- 

 phalic trunk, is derived by further seg- 

 mentation and division of the tracheal bulla. 



2. The second segment of the stem 

 bronchus is formed by the primary caudal 

 trunk of the primitive lung. 



3. The third segment is continued cau- 

 dad as the representative of the medial 

 secondary caudal branch of Hystrix and 



axidea. 



The general conclusions reached in the 

 paper may be summed up as follows : 



1 . The right and left lung agree morpho- 

 logically in the type of their bronchial dis- 

 tribution. 



2. The asymmetry, when observed, is ap- 

 parent, not real, depending usually upon 

 complete separation of the right primary 

 cephalic trunk into two components, the 

 proximal one of which changes its original 

 relation to the bronchial stem and pul- 

 monary artery by migration cephalad. 

 More rarely the asymmetry depends upon 

 the complete migration cephalad of the en- 



tire cephalic trunk carrying both secondary 

 branches {ifyrmecophaga) . 



3. Aeby's hypothesis of the morpholog- 

 ical equivalence of the middle right and 

 upper left lobe of the human lung is there- 

 fore untenable. 



The proposition should read : 



BIGHT LUNG. 



LEFT LUNG. 



Upper + middle lobe = upper lobe. 

 Lower + cardiac lobe = lower lobe. 



4. The active agent in changing and 

 modifying the architecture of the lung is 

 not the pulmonary a.rtery ( Aeby) , but the 

 migration of the cephalic primary trunk, or of 

 its proximal secondary derivative, usually 

 only on the right side, producing apparent 

 asymmetry. This migration affords an op- 

 portunity for more complete development 

 of the resulting terminal bronchial system, 

 and for consequent 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 indicated by the de- 

 velopment of a special accessary cardiac 

 bronchus of the right side. 



6. Except, therefore, for purposes of topog- 

 raphy, the distinction of Eparterial and 

 Hyparterial bronchi should be abandoned, 

 at least to the extent of clearly recognizing 

 the fact that in asymmetrical lungs, every 

 right ' eparterial ' bronchus finds its mor- 

 phological equivalent among the ' hypar- 

 terial ' bronchi of the left side. 



7. The impropriety of ascribing any 

 morphological significance to the number of 

 pulmonary lobes is apparent. The division 

 of the lung into lobes is an entirely second- 

 ary character, not dependent upon the type 

 of bronchial distribution, but probably con- 

 nected with the unequal mobility and rate 

 of expansion in different segments of the 

 thoracic walls. 



