96 



AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



diameter as they pass dorsally. This is seen in Hochstetter's figure and is mentioned 

 in Barniville's (1914) account of a human embryo. The sixth artery shows 

 much less slope, in both its proximal and distal parts, than the others. The 

 seventh, of which the subclavian is a branch, is a robust vessel and comes off 

 perpendicularly from the aorta. The reason for the dissimilarity between this 

 vessel and the more cranial segmentals can be better understood after an examina- 

 tion of a slightly older stage. 



In a second model of a 9-mm. embryo the vertebrals are midway in their 

 formation (fig. 28). Anastomoses are now established between all the cervical 

 segmental arteries except the first and second. A certain amount of variability in 

 the details of the formation of the vertebral is well illustrated by the equality in the 

 number of interrupted segmentals in this embryo and the other previously described, 

 taken with the dissimilarity shown by the two in the development of anastomoses. 



28 



Figs. 27 and 28. Stages in the formation of the vertebral artery. In figure 27 (embryo No. 721, 9 mm.), two seg- 

 mental arteries are interrupted but no anastomoses have yet formed between them. In figure 28 

 (embryo No. 143, 9 mm.), retrocostal anastomoses have formed between all but the first and second 

 segmental arteries. 



Also, it is not the same segmentals that are interrupted in the two embryos. The 

 individuality in the story of the interruption of the successive arteries is clearly 

 seen in their varying angles relative to the aorta and in the inequality of the intervals 

 between their origins. 



The anastomoses between the segmental arteries, with the exception of the 

 first and second, are of a regular character, since they are situated among seg- 

 mentally arranged vertebral nerves and arteries. They pass caudally from a 

 segmental vessel, where it lies medial to and indented by a spinal nerve, and connect 

 with a more distal part of the next succeeding artery. In this way a channel is 

 developed from alternating anastomoses and segments from successive segmental 

 arteries. The last contribute the larger amount to the vertebral artery. If the 



