282 Dr Allen Thomson on the Vascular System 



the circulation of the blood is tioSv (ibmpletely Established 'oiT the 

 vascular area, but before the second set of veirisMve appeared, 

 I have found the two aortic vessels united for a considerable 

 space in the dorsal region. This union seems to commence in 

 the' t)ack, nearly opposite tb tfte aiiricle, but I have tiot b^fi 

 afcle to ascertain the precise period at which this process begins: 

 it gradually extends backwards towards the tail, so that, at the 

 60th or 65th hour, the whole of the dorsal and part of the ab- 

 dominal aorta is one tube, as far as the place where the omphalo- 

 mesenteric arteries are given off. The omphalo-mesenteric ar- 

 teries, being shortly after this partially united, appear to arise 

 from one stem. ^^'' 



On the fourth day, the whole of the two abdominal portidrts 

 of the aorta becomes united, as far as the region where the per- 

 manent division of this vessel takes place : here the vessels ife- 

 main separate, and furnish the umbilical arteries or vessels of 

 the allantoid membrane, which now begins to be developed, — 

 these being the first considerable branches of the iliac arteries 

 which are formed. 



While this union of the dorsal and abdominal portions of the 

 double aorta takes place, the two vessels arising from the bulb 

 of the heart, of which the aortae formed at first the continua- 

 tion, do not, like these, become united into one trunk, as the 

 observations of M. Serres would lead us to believe. I have al- 

 ready described these two vessels * as the first pair of bran- 

 chial arches, the posterior parts of which form the separate 

 roots of the aorta to be found in the chick on the third and 

 fourth days of incubation ; these roots being also joined at this 

 period by the four other branchial arches which appear succes- 

 sively on each side of the pharynx. These roots of the aorta 

 and branchial arches, we have already remarked, do not become 

 united to one another, but undergo other very remarkable 

 changes, by their partial enlargement or obliteration. Parts 

 of the first branchial arches give lise to the carotid arteries in 

 all vertebrated animals : while the proper trunk of the aorta, or 

 at least its ascending portion and arch, is produced from other 

 branchial vessels, and the roots into which they are joined ; one 

 or more of these serving to form the aorta, according to the 



• See page 257, &c. and figs. 20, 21, and 30. in the last Plate. 



