THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 573 



lower jaws. The carotis interna (figs. 316 d, 317 c') likewise arises 

 from the first arch, but farther dorsally, at the point where the 

 arch bends around to become continuous with the root of the aorta ; 

 it conducts the blood to the embryonic brain and to the developing 

 eye-ball (arteria ophthalmica). From the dorsal region of the 

 fourth vascular arch (fig. 316 4 ) a branch is given off which is 

 soon divided into two branches, one of which goes headwards to the 

 medulla oblongata and the brain, the arteria vertebralis (v), whereas 

 the other (s) supplies the upper limb (arteria subclavia). In the 

 course of development these two arteries interchange relations in 

 respect to calibre. In young embryos the vertebralis is by far the 

 more important, while the subclavia is only a small inconspicuous 

 lateral branch. But the more the upper extremity increases in size, 

 the more the subclavia is elevated into the position of the main 

 trunk, and the more the vertebralis sinks to the rank of an accessory 

 branch. Finally, from the fifth [sixth] arch there bud forth branches 

 to the developing lungs (figs. 316, 317 p). 



As the simple diagram shows, the fundament of the arterial trunks 

 which arise from the heart is originally strictly symmetrical. But at 

 an early period there occur reductions of certain vascular tracts even 

 to their complete disappearance ; in this way the symmetrical arrange- 

 ment is gradually converted into an unsymmetrical one. 



The accompanying diagram (fig. 318) in which the parts of the 

 vascular course that degenerate are left free, and those which 

 continue to be functional are marked by a heavy central line will 

 serve to illustrate this metamorphosis. 



First, as early as the beginning of the nuchal flexure, the first 

 and second vascular arches with the exception of the connecting 

 portions through which the blood flows to the carotis externa (b) 

 disappear. 



The third arch (c) persists, but loses its connection with the dorsal 

 end of the fourth, and therefore now conveys all its blood toward the 

 head into the carotis interna (a), of which it has now become the 

 initial part. 



The chief role in the metamorphosis is assumed by the fourth and 

 fifth arches (fig. 317 (7). They soon exceed all other vessels in size, 

 and as they lie nearest to the heart, they are converted into the two 

 chief arteries which arise from it, the aortic arch and the arteria. 

 pulmonalis. An important modification is effected at the place of 

 their origin from the truncus arteriosus when the latter is divided 

 lengthwise by means of the development of the partition previously 



