THE CIRCULATION IN AMPHIBIA. 693 



the vena cava inferior. Retia mirabilia, both arterial and venous, exist in 

 certain Fishes, on the swimming bladder, in the eyes, and in the neighborhood 

 of the gills and the intestines. 



Since all the blood of Fishes, after its return from the body, passes through 

 the respiratory organs, before it again proceeds to the body, it might be in- 

 ferred that the respiration must be more complete in them than in the Rep- 

 tiles, in which only a portion of the venous blood is transmitted to the respir- 

 atory organs, whilst some is distributed to the body again. But in Fishes, 

 the respiratory process itself is less active than it is in Reptiles, being aquatic, 

 instead of aerial. As the heart in the Fish is a branchial or respiratory heart, 

 its single auricle and ventricle have been supposed to be homologous with the 

 right auricle and ventricle, or respiratory heart, of the Bird and Mammal. 

 This view, however, is only partially correct ; for though the auricle in the 

 Fish's heart, is homologous with the right auricle of the more perfectly formed 

 Mammalian or Avian heart, and therefore respiratory, the single ventricle 

 represents both ventricles of the heart of the warm-blooded Vertebrata, and, 

 indeed, rather the left than the right ventricle. In the Fish, the branchial 

 vessels given off from the arches, which proceed from the bulbus arteriosus, 

 end in the systemic arterial trunk, and the branchiae and their vessels may 

 therefore be regarded as organs interposed in the systemic circulation. In the 

 pulmonated Vertebrata, on the contrary, the pulmonary arteries enter the 

 lungs, from which the blood is returned through pulmonary veins, and these 

 latter do not unite to form an artery, but re-enter the auricular portion of the 

 heart ; so that the pulmonary system is not, like the branchial system, con- 

 tinuous with, or a portion of, the systemic, but altogether a special circulation. 



The Amphibia begin life as aquatic branchiated animals, but most of them, 

 in their mature state, lose their gills and acquire lungs, so as to become pul- 

 monated and air-breathing. In this metamorphosis, not only transitions 

 occur in the respiratory organs from the Piscine to the Reptilian condition, 

 but also simultaneous adaptations of the circulating system. The differences 

 in the mode of respiration, and in the character of the circulation, which are 

 observed between two great Classes of the Vertebrata, viz., Fishes and Rep- 

 tiles, are exactly paralleled by the differences met with in the immature and 

 mature respiratory and circulatory organs, in an individual Amphibian ; and 

 the progressive steps which lead from one state to the other, can be seen in the 

 evolution of a single animal. These changes may be traced in the frog, or in 

 the salamander. In the tadpole of the former, external branched gills first ap- 

 pear, but soon become absorbed, and are succeeded by internal laminated gills, 

 resembling those of the Fish. In this condition, the heart, composed of a 

 single auricle and ventricle, receives the blood from the body, and propels it 

 into a bulbous arteriosus, and thence, by three lateral symmetrical branches, 

 or branchial arterial arches, into the gills ; after passing through capillary ves- 

 sels, it is collected by the branchial veins, from the foremost of which the ar- 

 teries of the head are given off; these veins, but chiefly the second and third, 

 combine to form the systemic artery or descending aorta, which conveys the 

 blood into the rest of the body, whence it is again returned by the veins, to 

 the single auricle. This form of circulation is truly Fish-like ; but at the base 

 of each branchial lamina, a minute vessel runs directly from each branchial 

 arterial arch, to the commencement of the descending aorta ; as development 

 goes on, more blood passes through these communicating vessels, directly from 

 the bulb into the descending aorta, without traversing the vessels of the gills ; 

 still later, the gills themselves diminish, and more and more blood passes 

 through these little arched vessels at their base ; finally, the gills and their 

 vessels, being the one atrophied, and the other obliterated, the whole of the 

 blood proceeding from the ventricle through the arterial bulb, traverses these 

 enlarged symmetrical communicating vascular arches, and so reaches the de- 

 scending aorta. In the meantime, from the lowermost vascular arch, on each 

 side, there has been developed a little vessel, which ramifies on the walls of a 

 small sac, or rudimentary lung ; and as these organs are gradually enlarged, 

 the vessels in question grow, and form the pulmonary arteries. By these, one 

 portion of the blood from the single ventricle, is carried to the newly evolved 

 respiratory organs, the air-breathing lungs ; whilst the rest is conveyed 



