THE ORGANS OF THE INTERMEDIATE LAYER OR MESEXCHYME. 575 



to the right lung. On the left side of the body, on the contrary, 

 the pulmonary arch still persists for a long time and conducts 

 blood into the left lung and also through the cluctus arteriosus 

 Botalli (n), into the aorta. After birth, in connection with 

 pulmonary respiration, the duct of BOTALLI also degenerates. For 

 the lungs, when they are expanded by the first act of inspiration, 

 are in a condition to receive a greater quantity of blood. The 

 consequence is that blood no longer flows into the ductus Botalli, 

 and that the latter is converted into a connective-tissue cord, 

 which extends between aorta and art. pul- 

 nionalis. 



In addition to the regressive changes 

 mentioned, there are effected meantime 

 alterations of position in the large vascular 

 trunks that arise from the heart. They 

 move at the same time with the heart from 

 the neck region into the thoracic cavity. In 

 this fact lies the explanation of the peculiar 

 course of the nervus laryngeus inf. or re- 

 currens. At the time when the fourth 

 vascular arch still lies forward in the region 

 of its formation in the fourth visceral arch, 

 the vagus sends to the larynx a small nerve 

 branch, which, to reach its destination, 

 passes below [caudad of] the vascular arch. 

 When the latter migrates downwards, the 

 nervus laryngeus must thereby be carried 

 down with it into the thoracic cavity, and 

 must form a loop, one portion of which, 

 arising in the thoracic cavity from the vagus, 

 bends around the arch of the aorta on the 



left side of the body (but around the subclavia on the right side of 

 the body) to become continuous with the secDnd portion, which takes 

 the opposite or upward course to the region of its distribution. 



The processes of development discussed also throw light on a series 

 of abnormalities which are quite frequently observed in the large 

 vascular trunks. I shall cite and explain two of the most important 

 of these cases. 



Occasionally in the territory of the vessels of the fourth visceral 

 arches the original symmetrical condition is retained. The aorta is 

 then divided in the adult into right and left vascular arches, which 



Fig. 319. Diagrammatic re- 

 presentation of the meta- 

 morphosis of the arterial 

 arches in Birds, after 

 RATHKE. 



a, Internal, 6, external, 

 c, common carotid ; d, 

 systemic aorta; e, fourth 

 arch of the right side 

 (root of the aorta); /, 

 right subclavian; g, dorsal 

 aorta ; h, left subclavian 

 (fourth arch of the left 

 side) ; i, pulmonary ar- 

 tery ; k and I, right and 

 left ductus Botalli of the 

 pulmonary arteries. 



