502 SANGUIFEROUS SYSTEM. 



of external pressure to which the head and neck of birds are 

 exposed. They receive the blood chiefly from the external 

 parts of the head, and the oesophagus, crop, and other parts 

 of the neck, and, passing down superficially, along with the 

 pneumo-gastric nerve, they unite with the subclavian and 

 vertebral vein on each side, to form the right and left 

 superior venae cavse, as in the inferior vertebrata and most of 

 the lower mammalia. 



The venous blood thus conveyed to the right auricle 

 by the single inferior and the two superior venee cavee, is 

 sent by the right ventricle into the pulmonary artery, pro- 

 vided at its orifice with three semilunar valves, like the 

 aorta, which immediately divides into a right and left 

 branch, to ramify through the corresponding lungs. The 

 two pulmonary veins return the arterialized blood to the left 

 auricle of the heart by a single orifice, which is partially pro- 

 tected by a semilunar valve. The pulmonary arteries of 

 birds communicate at an early period with the aorta, by 

 means of the posterior pair of branchial arches, forming a 

 ductus arteriosus on each side, as in the lower pulmonated 

 vertebrata, and the latest of these ductus arteriosi to become 

 obliterated, is the one upon the left side, which also remains 

 alone to a late period in the foetal condition of mammalia. 

 The two brachio-cephalic arteries result from the metamor- 

 phosis of the anterior branchial or aortic arches of the em- 

 bryo, the descending aorta from that of the middle arch of 

 the left side, and the pulmonary arteries, as usual, from the 

 posterior aortic arch. So that the peculiarities in the struc- 

 ture of the heart of birds, and in the course of their sangui- 

 ferous vessels, are alike affinities to the lower reptiles and to 

 the inferior tribes of mammalia, and the metamorphosis of 

 their branchial arches is in accordance with that of all the 

 other pulmonated vertebrata. 



The same plan of structure observed in the vascular system 

 of the lower vertebrata, has arrived at its maximum of 

 development in the mammiferous animals, especially in the 

 higher quadrupeds and in man, but numerous modifications 

 of this complex hydraulic apparatus are necessarily presented 

 in this varied and extensive class, depending on differences 

 in the structure and condition of internal parts, or in the 

 general outward form of the body, or connected with pecu- 

 liarities in the living habits of species. The right and left 



