. Contributions to embryology. Embryology. 20 THE DEArELOPMENTAL ALTEUATIONS IN THE VASCULAR SYSTEM The main arterial trunks are well established at this stage and afford a more abundant supply 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 carot
. Contributions to embryology. Embryology. 20 THE DEArELOPMENTAL ALTEUATIONS IN THE VASCULAR SYSTEM The main arterial trunks are well established at this stage and afford a more abundant supply 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 exjiect, 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. SIN. TRANSVERSUS PLEXUS .MEDIALIS PLEXUS POST. SIN PETROS SUP. FORAMEN JUGULARE V OPTHAL Profile reconstruction of the flural venous in a human enihryo IS mm. lonK (CarneKie Collection, No. 144). The primary head-vein is still intact: a more dorsal channel, however, is formint; throiich the meshes of the middle dural plexus, coursing backward into the i)osterior plexus. This new channel becomes the permanent sinus tranavensus by which the sreater part of the brain is finally drained throush the foramen juRulare. A course completely in'crcranial (sinus transversus) thus replaces one that was in part extracranial (primary head-vein). Median to the second division of the trineminal nerve can he seen the plexiform maxillary vein, which drain.'* the stnict
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