THE CIRCULATION IN REPTILES. 691 



over, the chief branches from the arch of the aorta, are also symmetrical, in 

 accordance with the equal bilateral development, so important and typical in 

 Birds. The veins in Birds have relatively fewer valves ; the portal system 

 communicates, by a few branches, with the renal veins ; moreover, the veins 

 of the pelvis and lower limbs likewise contribute branches to it. 



In the cold-blooded, pulmonated Vertebrata, which comprise the Keptiles and 

 the perfect Amphibia, the heart is reduced to three cavities ; viz. , two auricles 

 and one ventricle. One auricle, the right and larger one, receives blood from 

 the system ; the other, the left, usually smaller, receives blood from the lungs ; 

 both discharge their contents into the common ventricle, which thus receives a 

 mixture of dark venous blood from the system, and bright arterial blood from 

 the lungs. From the single ventricle there proceed, in the Reptilia, a distinct 

 aortic and pulmonary trunk, but in the Amphibia, only a single arterial trunk 

 exists, from which the pulmonary arteries take their origin ; in either case a 

 part of the mixed ventricular blood again passes through the lungs, whilst 

 most of it is propelled through the body. The blood is no longer entirely sent 

 through the lungs, before it is distributed to the body again, as happens in 

 the warm-blooded Birds and Mammals, which possess a perfect double circu- 

 lation ; on the contrary, a more or less mixed blood goes to the lungs, and a 

 more or less mixed blood to the body. The pulmonary and systemic circula- 

 tions are not wholly distinct, but meet in the common ventricle ; the circula- 

 tion is not completely double. In the highest Reptiles, the Saurians, which 

 approach the Birds in so many parts of their organization, there exists, how- 

 ever, a partial ventricular septum, which, in the crocodiles, is said sometimes 

 to be even complete. The outwardly single ventricle always gives off two ar- 

 terial trunks, and the internal septum is so placed, that it serves to direct the 

 dark systemic venous blood entering from the right auricle, chiefly into the 

 pulmonary arterial trunk, whilst it turns the current of red arterialized blood 

 coming through the left auricle from the lungs, towards the aortic or systemic 

 arterial trunk. In these animals, then, the circulation approaches closely to 

 the character of the double circulation in Birds and Mammals ; but the im- 

 perfect structure of the ventricular septum may permit a certain amount of 

 intermixture of the two kinds of blood in that cavity. The right or anterior 

 portion of the ventricle, which is connected with the pulmonary artery, has 

 thinner walls than the left or posterior portion, which is connected with the 

 aorta. In the Chelonia and Ophidia, a less perfect septum also exists, but it 

 gradually becomes smaller, and therefore less able to separate the two currents 

 of blood, or to guide these, in special directions. In the perfect adult Amphi- 

 bia, the single ventricle has either only slight traces of a septum, or, more 

 commonly, no septum whatever ; the single arterial trunk proceeding from it, 

 is sometimes named the arterial bulb, or bulbus arteriosus. Even the auricular 

 septum is imperfect in the Proteus. 



The arrangement of the branches of the aorta in these cold-blooded pul- 

 monated Yertebrata, is also peculiar. In Man and Mammalia, the single 

 aortic arch bends over the root of the left lung, and continues on as the ab- 

 dominal aorta. In Birds, the arch of the aorta turns down over the root of 

 the right lung, to become the abdominal aorta. In the higher Reptiles, two 

 aortic arches exist, one on each side, forming a right and left aortic arch, 

 which descend over the roots of the corresponding lungs, and join together 

 somewhere in front of the vertebral column, to form the abdominal aorta. Of 

 these, the right aortic arch, the only one present in Birds, is larger than the 

 left, and evidently forms the proper systemic artery ; for it gives oft' the ar- 

 teries to the head, neck, and upper limbs, which parts accordingly receive 

 almost entirely red blood, which is directed into the aorta, as already men- 

 tioned, by the ventricular septum. On the other hand, the left aortic arch is 

 small, is joined by a short trunk to the pulmonary artery, which springs from 

 the other side of the septum, and from it receives dark blood ; hence it follows 

 that the abdominal aorta, formed by the coalescence of the right and left aortic 

 arches, carries mixed blood to the posterior part of the trunk and the hind 

 limbs. In the lower Reptiles three aortic arches exist on each side ; the upper 

 pair give off the vessels of the head and neck, the lower pair give origin to 

 the pulmonary arteries, and all combine, by short branches, into two descend- 



