. Carnegie Institution of Washington publication. 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 commo
. Carnegie Institution of Washington publication. 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 struc
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