20 



THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



The main arterial trunks are well established at this stage and afford a more 

 abundant supplj' of blood to the brain than existed in the 4 mm. embryo. Whereas 

 the aortic system in the latter conformed to the branchial arches, it now presents a 

 definite aortic arch derived from the truncus arteriosus and the fourth branchial 

 arch of the left side. The innominate artery is formed by the fourth arch of the 

 right side. The third arch has been taken up on each side in the formation of the 

 common carotid and its bifurcating portion, including the plexiform external 

 carotid. The internal carotid, basilar and vertebral arteries are present in prac- 

 tically the adult form. It was noticed in studying the left vertebral artery in this 

 specimen that its communication with the aorta was more caudal than one would 

 expect, judging by its position in the adult. It is probable, however, that the 

 arrangement is a temporary one, and that one of the communications above this, 

 too slender to model, is destined to become the final trunk of the vertebral artery, 

 thus affording an instance of spontaneous migration of a blood-channel. 



PLEXUS .MEDIALIS 



SIN TRANSVERSUS 



PLEXUS POST. 



SIN. 

 PETROS. SUP 



FORAMEN 

 JUGULARE 



FiGUBE 2. 



Profile reconstruction of the dural venous system in a human embryo 18 mm. long fC'arnegie Collection, No. 

 144). The primary head-vein is still intact; a more dorsal channel, however, is forming through the 

 meshes of the middle dtiral plexus, coursing backward into the posterior plexus. This new channel 

 becomes the permanent sinus transversus by which the greater part of the brain is finally drained through 

 the foramen jugulare. A course completely intercranial (sinus transversus) thus replaces one that was in 

 part extracranial (primary head-vein). Median to the second division of the trigeminal neri-e can Ije seen 

 the plexiform maxillary vein, which drains the structures of the maxillarv- process. Enlarged 13 diameters. 



Similarly, as was seen in the 4 mm. embryo, the distribution of the arterial 

 supply and the arrangement of the venous drainage are here (in the 14 mm. embryo) 

 efficiently laid out from the standpoint of the structures of the head as then exist- 

 ing. The primary head-vein is already somewhat bent out of its course by the 

 ear- vesicle and nerve-trunks, and would necessarily become much more so by the 



