236 STRUCTURE OF THE VERTEBRATES 



complete. The left radix, which is less functional than the right 

 in reptiles, has disappeared; the right remaining as the aorta 

 of the birds. The base of the left fom^th branchial (the left radix 

 of the reptiles) remains as the left subclavian artery, supplying 

 the wing and shoulder. 



Mammals are essentially like the birds in the modification of 

 the aortic arches, with the exception that the left radix remains 

 as the aorta. The left subclavian, like that of the more primitive 

 vertebrates, is a branch from the aorta, and the right subclavian 

 is connected with the aorta by the base of the fourth branchial 

 arch (the right radix of the reptiles). Following is a summary 

 of the degeneration of the aortic arches of the mammal. Recall 

 that in the embryo the ventral aorta is bifurcated from the 

 level of the third arch to its anterior end. Each half of the 

 anterior ventral aorta extends to the head as the external carotid, 

 and the anterior portion of the dorsal aortae form the internal 

 carotids. The common carotids are formed by the divided region 

 of the ventral aorta between the third and fourth arches. 



Arch I, disappears. 



Arch II, disappears. 



Arch III, connects the common carotid with the internal 

 carotid. 



Arch IV, forms: (1) the leit gives rise to the "aorta"; and 

 (2) the base of the right connects the aorta with the subclavian. 



Arch V, disappears. 



Arch VI, forms the pulmonary artery. The dorsal half disap- 

 pears, destroying the connection between the pulmonary and 

 the aorta. 



4. The Cardiac Cycle of the Mammal. The foetal circulation 

 of the mammal differs widely from that of the adult. The lungs 

 are non-functional, respiration taking place in the placenta. Con- 

 sequently, aerated blood comes into the heart through the veins, 

 mixed with unaerated. Blood is carried back to the placenta 

 through branches of the dorsal aorta, with the result that blood 

 within the embr^^o is always mixed. 



Blood is prevented from going to the unexpanded lungs by 

 three mechanisms: (T) as the blood pours into the foetal auricles, 

 most of that which falls into the right auricle passes through the 

 opening in the auricular septum (foramen ovale) and goes into 



