60 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



influence of the blood-vessels by the fact that the grooves on the ventral floor of 

 the pharynx, filled for a time by the first and second aortic arches, do not disappear 

 when these vessels are lost. 



An entirely different explanation for the presence of the sac has been suggested 

 by Dr. Streeter. He has observed that it is a characteristic of early vessels, well 

 illustrated by the early dorsal aorta, to have a diameter much greater proportion- 

 ately than would be required for the adult vessel. He suggests that this may be 

 due to proliferation of reserve endothelium which a little later will be used in the 

 rapid differentiation of the vascular system. 



The connections of the aortic arches with the arterial or aortic trunks are 

 termed paired ventral aorta? in most text-books of human anatomy, and the schemata 

 which they contain correspondingly show the arches arising from a pair of longi- 

 tudinal ventral trunks. As has been stated, a few investigators have recognized 

 the error of this description by using the term bulb or swims. While the arterial 

 blood in the human embryo passes from trunk to arch by an unpaired sac, there are 

 certain temporary channels to single arches which, by their cranio-caudal course, 

 resemble fragments of a ventral aorta. Such are the longitudinal ventral segments 

 which appear in the later history of the first and second arches and the paired 

 ventral sprouts which for a time run caudally from the pouch before they take on a 

 more transverse direction as part of the pulmonary arches. One might even 

 include the primitive ventral arterial twigs of the subpharyngeal regions, which 

 have the position of ventral aortse in the region of the first and second aortic 

 arches, though at a time when the arches have already disappeared. These 

 various more or less longitudinal elements are rightly to be regarded as indications 

 of a general structural plan common to higher and lower vertebrates, but carried 

 on in some of the latter to a completeness which admits of the existence of paired 

 ventral aortae. However, these considerations certainly offer no justification for 

 the use of the term ventral aortce in man, since such vessels are not to be found at any 

 stage of his development. 



INVOLUTION OF FIRST AND SECOND AORTIC ARCHES ORIGIN OF STAPEDIAL AND 



EXTERNAL CAROTID ARTERIES. 



In the region below the propharynx there is a period of instability and of 

 readjustment of the vascular channels after the disappearance of the first and 

 second aortic arches. Our study of this period is based on but few models, since 

 only vessels turgid with blood or good artificial injections can be relied on to demon- 

 strate the change of the arches into a plexus and the beginning of the arteries 

 therefrom. 



Soon after the third arch is established, the first has given place to a tortuous 

 and much more slender channel (fig. 3). It is best developed at the upper end of 

 the visceral arch and is usually lost in the plexus at the lower end. There is often 

 distinguishable close to the vestibule an arterial sprout occupying the position of 

 the ventral end of the arch before its disappearance (figs. 3 to 9). After the fourth 

 arch is complete, a similar channel is found to have replaced the second arch; 

 this also is usually lost in a plexus in the subpharyngeal region. These vessels are 



