AORTIC ARCHES. 793 



ing to a large size along with the growth of the placenta. As the 

 limbs are formed, arteries are developed in them, and these are branches 

 of the main aorta ; but they are for a long time comparatively small, 

 w'hile the umbilical arteries speedily attain a large size, so that, even up 

 to the conclusion of foetal life, they appear to form the principal part of 

 the two large vessels into which the aorta divides at its lower extremity. 

 The middle sacral artery may be looked upon as the continuation of the 

 median stem of the aorta, and probably originates from a double vessel 

 in the same manner as the aorta itself. 



The double state of the main aorta when first formed in the foetus was dis- 

 covered by Serres, and described bj him in his 4tli Memoir on Transcendental 

 Anatomy (Annal. des Scien. Nat., 18130), but was doubted by Von Baer, as 

 Serres's observations did not show the relation of the primitive trunks of the 

 aorta to the pharyngeal vascular arches. The fact of the original double condi- 

 tion was, however, placed beyond doubt by Allen Thomson (Edin. New Philos. 

 Journal, ISoO) by the method of tranverse sections, then fii-st employed as a 

 means of embryolog'ical investigation, and the process of median union was 

 traced in full detail. The relation of this process to the occurrence of a perma- 

 nent double canal in the aorta as a malformation, as described by Vrolik. Schroder 

 van der Kolk and Cruveilhier. and obseiwed in at least one case by Allen Thomson, 

 has already been refeiTed to in vol. i., p. 350. 



According- to Serres, the vertebral arteries within the cranium are originally 

 separate, and the basilar artery results from their mesial union or fusion in the 

 same manner as occurs in the aorta, and the median union of the anterior cere- 

 bral arteries in the forepart of the Cu'cle of Willis is another example of the 

 same process. It seems probable that the internal cross band obser%^ed liy John 

 Davy in the interior of the basilar artery (Researches Physiol, and Anatom., 

 lSo9, p. 301) may be a remains of the septum or united walls of the two vertebral 

 arteries. 



Aortic OX' Branchial Arclies. — The two primitive arterial arches 

 which lead into the dorsal aorta from the arterial bulb of the rudi- 

 mentary heart, at the time of the establishment of the first circulation, 

 are the most anterior of a series of five pairs of vascular arches which 

 are developed in succession round this part of the pharynx ; and which, 

 since their discovery by Eathke in 1S25 (Oken's Jsis, 1825) have been 

 regarded with much interest, as coiTcsponding with those vessels which 

 are the seat of development of the subdivided blood-vessels of the gills in 

 fishes and amphibia. These vascular arches thus exhibit in the amniota, 

 along with the branchial or pharyngeal clefts and visceral plates, a typical 

 resemblance to the structure of gills, although no full development 

 of these respiratory organs occurs in such animals, but they furnish by 

 their various transformations the basis of formation of the permanent 

 pulmonary and aortic stems and the main vessels to which they give 

 rise. 



The form and position of the primitive aortic arches, up to the time 

 of their transformation into permanent vessels, is nearly the same in 

 reptiles, birds and mammals ; and the main differences in the seat and 

 distribution of the large permanent vessels are to be traced to changes 

 in the openness and extent of growth of the several arches. The five 

 pairs Of arches do not all co-exist at the same time, for they are deve- 

 loped in succession from before backwards, and by the third day of 

 incubation, or by the corresponding period of the fourth week in the 

 human embryo, when the posterior arches have been formed, already a 

 part of the anterior arches, beginning with the first one, has become 



