John Lewis Bremer 



139 



of blood in the unused lungs. The left pulmonary arch grows rapidly 

 in diameter as well as in lengtli, while the right becomes entirely oblit- 

 erated beyond the point where the pulmonary artery arises, leaving 

 finally no trace of its existence; from this point to the junction with 

 the left arch to form the truncus pulmonalis, the right arch remains 

 of the same calibre as the pulmonary artery. The small twist marking 

 the origin of the pulmonary artery gradually straightens out, and the 

 whole right side, i. e. the anterior portion of the fifth arch and the 



Fig. 3. 



Fig. 4. 



Fig. 3. Habbit of 10.0 mm. Frontal series No. 157. sections 347-367. 

 X 80 diameters. , 



Fig. 4. Pig- of 7.8 mm. Frontal series No. 430, sections 270-297. X 80 

 diameters. 



pulmonary artery, being now unattached to the right dorsal aorta, is 

 drawn to the left by the larger left aortic arch, which is constantly tend- 

 ing to become straight. As a result of these changes, the left pulmonary 

 arch seems to give rise, at about its mid-point, to two arteries, with their 

 origins close together (or there may be a very short common stem); the 

 right one, the longer of the two, arising anteriorly, and taking its course 



