in the Foetus of Vertebrated Animals. 259 



sent through the anterior branches from the bulb of the aorta, is 

 carried to the carotid and brachial arteries exclusively, and no 

 longer reaches the root of the aorta. There now remain, there 

 fore, only four vascular arches, viz. the fourth and fifth pairs 

 (p r), from which the proper trunks of the aorta and pulmonary 

 arteries are formed. 



It has already been shewn, that, on the fifth day, the septum 

 of the ventricles is completed, and separates entirely the right 

 from the left arterial cavity of the heart. These cavities during 

 their contraction propel their contents into the bulb of the aorta, 

 which remains a single cavity for some time after the ventricles 

 are distinct. Towards the ehd of the fifth or beginninfj of the 

 sixth day, according to Baer, the bulb of the aorta becomes flat- 

 tened, and the opposite sides of the tube adhere together along 

 the central part, so as to separate it into two vessels, situated 

 side by side, and enclosed within the same sheath. The vessel 

 situated on the left side, becoming considerably shorter, after- 

 wards forms the root of the pulmonary arteries ; that on the 

 right side forms the commencement of the aorta. As these ves- 

 sels join their respective ventricles, they appear to cross, or to 

 be twisted round one another. 



As development proceeds, the root of the aorta is more com- 

 pletely separated from that of the pulmonary artery, and a re- 

 markable change, at the same time, takes place in the direction 

 of the blood through the vessels into which they lead. The 

 blood, propelled by the contraction of the left ventricle through 

 the aortic root (R), instead of going as before into all the vas- 

 cular branches rising on the fore part of the neck, now passes 

 only into the fourth vascular arch on the right side, and the two 

 arteriae innominatse which arise from it (#, u, r, s) ; while the 

 blood from the right ventricle is sent through the pulmonary 

 root (P) exclusively into the fourth arch on the left side, and 

 the two fifth or posterior arches {p, p) *. The fourth arch 

 now becomes gradually larger, and becomes the freest mode of 

 communication between the ascending and descending aortae ; it 

 forms, in fact, die proper arch of the aorta. In the mean time, 



• Baer explains this change by supposing that, when the roots of the pul- 

 monary artery and aorta are separated, the blood acquires a new direction, 

 and is thus driven into their respective vessels. 



