AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 91 



(fig. 20). This is due to the fact that the part of the vessel distal to the vertebral 

 artery shares the movement of the ribs, while the proximal portion is held to its 

 original position by the vertebral artery and other structures. The entire sub- 

 clavian now lies in nearly the same transverse plane, since its origin is about level 

 to the upper surface of the first rib. 



The left subclavian stem in the early post-branchial period continues its earlier 

 ascent along the aorta and on up the aortic arch. It was seen, in following the 

 development of the arch, that this is truly a process of moving of the subclavian 

 relative to the wall of the parent trunk and not a mere shortening of the aortic 

 segment proximal to its origin. From the relation of the proximal end of the 

 vessel to the definitive aortic arch, just when the rapid descent is reaching its 

 completion in embryos 21 to 24 mm. in length, it is clear that this shifting is not 

 rapid enough to compensate entirely for the aortic descent. At this time the 

 subclavian is still separated at its origin from the left common carotid by a con- 

 siderable segment of the arch, with which it makes an acute angle. In Jackson's 

 models of a 31-mm. and a 65-mm. human embryo, copies of which are manufactured 

 by Hammar, the subclavian is shown already lying close to the left common carotid 

 artery (figs. 16 and 22 to 25). 



BASILAR ARTERY. 



In the earliest work on the development of the basilar and vertebral arteries, 

 His (1880) made the error of regarding the two vessels whose fusion produces the 

 basilar artery as vertebral in nature and fixed this idea in the literature by designat- 

 ing them the cephalic vertebrals. Macalister (1886) concluded, apparently from 

 a study of the chick, that these vessels are not homologous to the vertebrals but 

 correspond to the system of vessels running along the surface of the nerve tube. 

 De Vriese (1905) confirmed this in the rabbit. The so-called cephalic vertebrals 

 of His are continuous with similar vessels along the anterior surface of the spinal 

 cord. Sabin (1917) also has studied these vessels by the injection method. In 

 chicks of about 27 somites she traced them from the subthalmic region to the 

 caudal end of the cord and termed them the longitudinal neural arteries. She 

 finds that they arise by the meeting of a prolongation of the internal carotid with a 

 cranial extension of an anastomosis of segmental arteries under the midbrain. 

 De Vriese claims that very early branches are found which extend from the proximal 

 part of the internal carotid up to a point on the two neural arteries cranial to the 

 part formed by anastomosis from the segmental arteries. It is possible that these 

 also take part in the anastomosis, giving rise to the caudal part of the longitudinal 

 neural arteries. 



The character of the paired longitudinal neural arteries is apparently somewhat 

 dissimilar in different species and perhaps also between the cerebral and the cord 

 regions in the same type. Sabin describes them as originating in the pig and the 

 chick in the form of a plexus on either side of the subthalamus and more definitely 

 as a pair of single channels along the rest of their course. In her figures they appear 

 not as a thickened band of plexus but as well-defined vessels. Sterzi (1904), using 



