ANATOMY OF A 17.8 MM. HUMAN EMBRYO 75 



arteries from the dorso-lateral aortae, later becoming temporary 

 branches of the internal carotids. 



The dorso-lateral aortae have ceased to exist as complete 

 trunks, for their continuity has been interrupted in the segment 

 of each intervening between the third and fourth aortic arches. 



The segm.ents cephalad of the third pair of aortic arches con- 

 sist of two symmetrical vessels which enter into the formation of 

 the internal carotid arteries, as already stated. The segments 

 caudad of the fourth pair of arches, consist of two very unsym- 

 metrical vessels which converge and unite, opposite the second 

 pair of thoracic nerves, to form the dorso-median aorta (Ao.d.m.). 



The caudal segment of the right dorso-lateral aorta is still 

 large in the neighborhood of the fourth aortic arch, with which it 

 participates in the formation of the right subclavian artery 

 (A.scl.d.). The remainder of this segment of the right dorso- 

 lateral aorta is reduced to a fibrous cord containing only a trace 

 of a lumen. The caudal segment of the left dorso-lateral aorta 

 is large and contributes to the formation of the permanent arcus 

 aortae. 



Although the intermediate segments of the dorso-lateral aortae 

 have lost all connection with the third pair of aortic arches, they 

 still persist as a pair of vestigial arterial tubes (*S.) projecting 

 from the cephalic aspect of the fourth aortic arches. 



The fourth aortic arches extend from the place of origin of the 

 common carotids, upon the right and left sides, to the vestigial 

 tubes above mentioned. They are both short, wide trunks, of 

 which the right occupies a more cephalic position. The right 

 fourth aortic arch eventually forms the proximal part of the right 

 subclavian artery. The left participates in the formation of the 

 arcus aortae. 



Pulmonary arteries. The truncus pulmonalis {Tr.puL, plates 1, 

 2 and 4) begins in the conus arteriosus (Con.art.) of the right 

 ventricle which is almost completely separated from the tnmcus 

 aorticus (vestibule) . Of the pulmonary arches, only the left per- 

 sists in its entirety. The truncus pulmonalis is now continued 

 directly into the left pulmonary arch which opens into the left 

 dorso-lateral aorta in common with the left fourth arch. 



