AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 95 



mammaries and the inferior epigastrics of the trunk and the caudal portion of the 

 longitudinal neural arteries of the brain arise in the same general manner, the 

 close association of the anastomoses of the forming vertebral arteries with the 

 vertebrae and other segmental structures gives them an unequaled regularity as to 

 position and form. In the development of the vertebral, the capacity of the blood- 

 current to make a channel for itself by converting to its use segments of many 

 different vessels is well shown and illustrates the hard and fast boundaries which 

 the environment may place upon the course of a blood-stream. This mode of 

 origin was already recognized for the vertebral by His in 1880. Froriep (1886), 

 in connection with his description of the development of the vertebral column of 

 the calf, showed the nature and relationships of the anastomoses. He figures them 

 as passing from one cervical segmental artery to the next through an opening 

 between the costal and the dorsal elements of the rudiment of the transverse 

 process of the vertebra and lying medial to the spinal nerve. Hochstetter (1890) 

 described a stage in the rabbit in which the anastomoses were large and swollen 

 and the segmental arteries still complete. He noted that the proximal end of these 

 vessels had been bent caudally by the shifting of the aorta in that direction. 



The vertebral arteries come into being because the cervical segmentals are 

 involved in the shifting of the neck structures on each other. The shifting of the 

 cranial end of the nerve-tube relative to the digestive tube and other more ventral 

 structures results in the cervical spinal ganglia and the spinal nerves after a while 

 taking an oblique course ventrally and caudally. The segmental arteries also 

 take on a similar direction (fig. 27). It is not certain whether the vessels would in 

 time have become modified to allow the arterial current to pass upward more 

 perpendicularly or whether the obliquity might have been permanently maintained. 

 In any case a distinctly unfavorable condition develops for the segmental vessels 

 at their proximal end due to the caudal shifting of the aorta. This affects only a 

 short segment of the artery, since the more distal part is held in its intersegmental 

 space by the condensed mesenchyme of the vertebral rudiments. As a result, 

 the short proximal part takes on a much more oblique direction than the rest. 

 A model of a 9-mm. embryo shows well this condition. 



In the proximal region of their abrupt slope the second and fifth cervical 

 segmental arteries form an angle of about 45 with the long axis of the aorta. In the 

 others of the series up to the seventh the slope is somewhat less. The segmentals 

 are evidently exposed to unusual longitudinal tension in this region. The abrupt 

 bending at either end of it, and more especially where it emerges from the aorta, 

 must tend to greatly retard the current-flow. The vessels are here under conditions 

 very unfavorable for further development or for even maintaining themselves. 

 In this part of their course they are all of very slender caliber, or, at the end closest 

 to the aorta, have become lost in the capillary plexus. It seems probable that their 

 involution consists in a distribution of the endothelial cells of their wall among 

 capillaries of the plexus that succeeds them and not in a cell degeneration. The part 

 of each segmental distal to the bend is of much greater diameter, and, taken as a 

 whole, the vessels have a characteristically conical form due to an increase in 



