AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 59 



larvae. It is also encountered in certain adult gill-bearing vertebrates. Rose 

 (1890) figures it in his study of the heart in the ganoid Polypterus bichir and the 

 urodele amphibian, Sieboldia maxima (Cryptobranchus japonicus). Dr. Harold 

 Senior tells me that in the American form, Cryptobranchus alleghenesis, the 

 enlargement is present, but the common cavity is much restricted by the medial 

 extension of septa between the openings of the arches. 



His (1880) and Bujard (1915) have recognized the existence of a ventral 

 aortic swelling in the human embryo and designated it aortic bulb. Gage (1905) 

 and Jordan (1909) termed it the aortic simis. Griel and Rose did not devote 

 especial attention to the sac in their studies of gill-bearing vertebrates and gave 

 it no name. In the adult fish and amphibian it is doubtless to be classed as an 

 aortic bulb, though these non-muscular enlargements distal to the heart do not 

 usually give off the arches directly. In this paper the specific term aortic sac 

 (saccus aorticus) will.be used for the embryonic enlargement. This is meant to 

 include not only the chambers between the arterial trunk and the arches, but 

 also the reduced sac distal to the aortic trunk, which persists for a time after 

 the pulmonary trunk has become separated off. 



On looking for an explanation for the expansion at this point it is necessary 

 to determine the relative importance of adaptation to function, such as is 

 found throughout the adult circulatory system, and of factors peculiar to the 

 developmental period. The aortic bulb of adult fish and amphibia probably 

 shares with the elastic mammalian aortic arch and other large arteries the function 

 of distributing the systolic pressure over a large portion of the arterial cycle. 

 Stahel (1886) claims that an enlargement of the portion of the human aortic arch 

 opposite the emergence of the innominate, carotid, and subclavian arteries is a 

 response to the added strain on the wall at this point resulting from the sudden 

 deflection of part of the current into these vessels. Thoma does not accept this 

 explanation. It is possible that the embryonic aortic sac is the result of the 

 combined action of these two principles. Yet it must be remembered that the 

 embryonic chamber differs greatly in its nature from the adult bulb and arch. 

 As to its makeup, we can say with certainty only that it consists of an endothelial 

 sac, though histogenetic study may well show that myoblasts- and fibroblasts are 

 already to be reckoned with. In any case its wall is very thin. It follows the 

 relief of the ventral pharyngeal wall; it is a cast of which the pharyngeal surface 

 is a mold. If we are to consider the embryonic sac as serving as an elastic reservoir 

 similar to the adult bulb and aortic arch, it is necessary to recognize the support 

 afforded by the pressure of surrounding resistant organs, exerted through the 

 intermediate mesenchyme, as, for example, the pharyngeal endoderm above and 

 the atria of the heart below. 



Kingsbury (1915a) noted that the arterial channels ventral to the pharynx, 

 including the aortic arches, fitted snugly into concavities of the pharyngeal wall, 

 and he concluded that the vessels exerted a molding influence upon it. It is 

 difficult to say just how much of the channeling of the phaiyngeal surface is due 

 to the arteries and how much to other factors. Doubt is cast upon a preponderating 



