SANGUIFEROUS SYSTEM IN THE VERTEBRATA. 87 



illustrated in their circulatory system : beginning life with the 

 single heart of a fish, and ending it with the double heart of a 

 reptile. In the water salamanders the venous blood which has cir- 

 culated through the body, is returned to a systemic auricle, and 

 having passed through the ventricle, it is received by the bulbus 

 arteriosus, and sent by the branchial arteries to the branchial 

 leaflets. The pure blood is now received by the pulmonary veins, 

 the confluence of which constitute, as in fishes, the descending 

 aorta. 



From this latter vessel a small branch passes off to the rudiment- 

 ary lung, which is afterwards to become the pulmonary artery. As 

 the animal changes from an aquatic to an atmospheric respiration, 

 the branchiag become absorbed, and the lungs proportionally deve- 

 loped. Their arteries experience corresponding changes, those of 

 the former organs diminishing, while those of the latter increase 

 with the growth of the lung. The two veins which return the 

 blood from the rudimental lung also enlarge, and as they arrive at 

 the heart, they undergo a remarkable dilatation, which constitutes 

 the left auricle. Till lately the bi-auricular form of heart was sup- 

 posed to be confined to the caducibranchiate amphibia, as frogs, 

 toads, salamanders, and tritons, but the researches of Owen have 

 proved its existence in the perennibranchiate amphibia also. 



At the same time that the systemic auricle receives the impure 

 blood from the cavae, the pulmonic auricle receives the aerated 

 blood from the lungs. From both of these cavities the blood is sent 

 into, and mixed in, the single ventricle, from which it is sent by 

 the one impulse to the lungs and to the system generally. From 

 this description it is obvious that the blood is but partially purified, 

 mixed blood being sent through the pulmonary arteries, as well as 

 through the aorta and its ramifications. 



REPTILIA. 



In this class of animals we perceive a still higher grade of deve- 

 lopment than was met with in the amphibia; the ventricle is par- 

 tially divided by a septum into two compartments, corresponding 

 in most particulars to the two ventricles of warm-blooded animals. 

 In some the septum is so imperfect as to be incapable of preventing 

 the admixture of the blood derived from both auricles. In others, 

 however, as the crocodile, the ventricles are separated completely, 

 or communicate by a small orifice provided with a valve, which 

 prevents the blood passing from one compartment to the other. In 

 fact the heart in this singular animal is double, as in the higher 

 vertebrata, so that the venous blood returned by the cava? to the 

 right auricle, passes from the right ventricle through the pulmonary 

 artery to the lungs, while the pure blood returned from this organ 

 to the left auricle is directed from the left ventricular compartment 

 through the systemic arteries. The auriculo-ventricular orifices are 

 provided with a muscular valve, and in the crocodile there are two. 



