AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 97 



proximal ends of the segmentals are drawn up into the vertebral, as in the case of 

 the interrupted divisions of the dorsal aorta, then a still larger fraction of its 

 material must come from the segmentals. Corresponding to the oblique course 

 of the latter, the vertebral artery presents a wavy or zigzag appearance when seen 

 from the side and front. A spinal nerve lies in each laterally open notch. For 

 some reason that was not determined, the anastomoses arch laterally for the 

 short distance in which they are free of the spinal nerve. 



We know from the course of the adult vertebral artery lateral to the anterior 

 branch of the suboccipital nerve that the anastomoses between the first and second 

 cervical segmental arteries must have this relation to the nerve, though more 

 caudal anastomoses lie medially. The anastomosis at this point could not be 

 followed from one artery to the other in any model, but it is evidently forming. 

 It is not clear just why it passes lateral to the nerve. The environment of the 

 first segmental differs from the surroundings of the others in that the spinal ganglion 

 and nerve are small and separated by a wide gap from the next of their series. 

 Other structural conditions connected with this and not so easily distinguishable 

 evidently permit the arching of the anastomosis to reach its maximum at this 

 segment. 



The first cervical segmental arteries, as noted in the discussion of the basilar 

 development, remain in connection with the paired longitudinal neural arteries 

 after the more cranial segmentals have begun to separate from them. As they are 

 still continuous with them at the time of formation of the vertebral artery, they 

 serve to continue the vertebral of either side into the basilar. It was not deter- 

 mined whether or not a short caudal segment of the longitudinal neural arteries 

 of the brain remains unfused on either side, but if it does it would furnish material 

 for the distal end of the corresponding vertebral artery. 



The vascular plexus on the lateral surface of the spinal cord and caudal end 

 of the medulla undergo striking changes during the establishment of the vertebrals. 

 Before anastomoses have developed we find the vessels turgid in the region of the 

 first cervical ganglion. The segmental artery here is also much distended where 

 it sends ramifications into the plexus of the medulla and cord. The distension of 

 the plexus and segmentals soon extends farther caudally along the cord. This 

 distension is perhaps due to the plexus and the distal part of the segmental arteries 

 temporarily carrying the blood-stream which was formerly distributed to it under 

 lower pressure from the arteries and which is soon to be taken over again by the 

 vertebral artery. Though the walls of the capillaries have not as yet been shown 

 to differ in strength from those of the segmentals, it is safe to assume from functional 

 considerations that they are already weaker. It is to be expected, therefore, that, 

 when the current of supply to the cervical cord is rather abruptly thrown directly 

 into the plexus, the walls of its capillaries and of the distal ends of the segmentals 

 as well should become stretched. 



There is a caudal decrease in distention of the vessels and in the size of the 

 anastomosis and a proximal tapering of the individual segmentals, which suggest 

 that, while the vertebral is forming, the blood-stream to the more cranial part of the 



