EPARTERIAL BRONCHIAL SYSTEM. 145 



If we seek for an explanation of the cause which leads to the 

 migratory changes of the cephalic bronchus, I admit that we 

 enter the realm of pure hypothesis. At the same time, the very 

 general development throughout the mammalia of this type, 

 with the resulting greater respiratory area of the right lung, may, 

 I think, not improbably be referred to the development of the 

 mammalian form of the systemic and pulmonary arteries. The 

 fact which seems to me to be most significant in this respect is 

 the development of the fourth and fifth embryonic arterial arches 

 (PL XXVIII). 



We know that with the septal division of the arterial trunk 

 into systemic aorta and pulmonary artery the fifth arches on 

 each side are assigned to the development of the latter vessel,^ 

 while the remaining arches are partially used in the elaboration 

 of the adult arterial system. 



If we consider the significance of the fcetal pulmonary incula- 

 tion it will appear at once that the conditions differ on the right 

 and left sides. 



On the left side the greater quantity of the blood thrown from 

 the right ventricle into the left pulmonary artery passes through 

 the Botallian duct directly into the aorta, only a small portion 

 traversing the left pulmonary circulation. 



On the right side, however, with the early obliteration of 

 the dorsal segment of the fifth arch, all the blood entering the 

 right pulmonary artery is forced to traverse the entire pulmonary 

 circulation, returning to the left auricle by the pulmonary veins. 



I believe that we may properly ascribe to this foetal circulatory 

 condition a great share in the more marked development of the 

 right as compared with the left lung. 



This view is further supported by the conditions found in 

 cases of " situs inversus," where the left lung develops the '* epar- 

 terial " bronchus (Lit. 6, 8, 9). 



^ It seems preferable, in general considerations, to disregard the existence of the 

 sixth arch, demonstrated by Boas and Zimmerman, on account of the extremely 

 temporary and evanescent character of the interpolated arch. 



