98 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



cervical cord is now coming by way of the basilar and internal carotid arteries. 

 Sabin's observations, from injections in the chick, that the paired longitudinal 

 arteries of the brain are formed by the meeting of a cranial branch from the in- 

 ternal carotids and a caudal vessel formed by the anastomosis of segmental arteries 

 is reason for assuming that at the time of the establishment of the longitudinal 

 neural arteries, at least, the carotid current passes backward under the fore part of 

 the hindbrain. The comparative anatomical studies of De Vriese have shown that 

 the supply of a large part of the brain by the vertebral current is an acquirement 

 of higher vertebrates. The internal carotids primitively reach the hindbrain. 

 De Vriese believed that she could trace in sheep embryos a progressive change in the 

 direction of tapering of the basilar artery of such a nature as to indicate that this 

 vessel at first acted as a branch of the carotids but later as a part of the vertebral 

 system. 



It is difficult to establish differences in diameter of vessels at this early stage 

 because of their great distensibility, dependent upon conditions at death and later. 

 There was, however, some evidence that the diameter of the vertebral at the time 

 of formation was greater in its more cranial part. It seems probable, therefore, 

 from these various considerations, that before the formation of the vertebral artery 

 the hindbrain receives its chief supply from its cranial connections and that during 

 the formation of the vertebral the current may pass back into its territory. The 

 establishment of the vertebral must sooner or later put an end to this condition. 



In the 14-mm. embryo the vertebral artery has acquired a nearly uniform 

 caliber (fig. 23). The proximal end of the subclavian, which was earlier the stem 

 of the seventh cervical segmental, has enlarged to the proper dimensions to carry 

 both the vertebral and the subclavian streams. The vertebral now comes off very 

 close to the aorta and it is distinctly larger than the subclavian. It is still nearly 

 as tortuous as at first. The maintenance of this condition in an artery for a con- 

 siderable time is of great rarity, since all vessels tend to straighten out their angles 

 rapidly. It persists only because its surroundings force this course upon it. In a 

 21 -mm. embryo (No. 448) the vessel is becoming straighter (fig. 25). 



The vertebral artery is formed by the elaboration of material from so many 

 sources that it will be well to enumerate its components. Beginning at its origin, 

 there is a segment of the seventh cervical segmental artery distal to the origin of 

 the primitive subclavian artery. Next come short portions of segmentals 1 to 7 

 with the anastomoses between them. The first cervical segmental carries on the 

 vertebral channel from the lateral side of the first cervical vertebra to the caudal 

 ends of the ventral neural arteries of the brain as they he under the medulla. As 

 has been said, it is not certain whether the ventral neural arteries themselves 

 contribute to it. 



The chief cause of the interruption of the segmental arteries has been given 

 as the tension of their proximal segments and their abrupt bending at each end 

 due to shifting of surrounding structures. A further understanding of the changes 

 ending in the vertebral formation may be gained from the seventh segmental, 

 which did not succumb to these conditions. There are two circumstances which 



