INTRODUCTORY OBSER VATIOJVS. 81 



are ultimately added to tlie animaFs organization, arc ex- 

 tremely small and feeble, serving only for awkward and im- 

 perfect progression on land, or along the bottom of tlie water. 



The organs of circulation undergo a no less striking and 

 far more important change. Tn the first instance the heart 

 is single, but the circulation is complete. The blood, after 

 its systemic circulation, is received from the veins by a single 

 auricle, and is immediately propelled by a fleshy ventricle to 

 the branchial arteries, of which one goes to each leaf of the 

 gills or branchia?. From the branchial capillaries, the aerated 

 blood is received by the branchial veins, which unite to form an 

 aorta without the intervention of a second ventricle. Every 

 tyro in comparative anatomy will at once perceive that this is 

 an exact description of the circulation in fishes ; and yet it is 

 taken from that of the Tadpole of the Newt. I shall not enter 

 here into a detail of the changes which take place in the various 

 vessels, by which the branchial vessels become obliterated or 

 altered in their course, and minute branches are augmented 

 in volume, and enter upon new functions ; such details could 

 only be understood by the experienced anatomist, and would 

 be out of place. It is sufficient to say, that by the dilatation 

 of one vessel a second auricle is produced ; that from the last 

 branchial artery a small branch passes to the air sac, or ru- 

 dimentary lung, which ultimately becomes the pulmonary 

 artery ; and that by other no less astonishing alterations, the 

 transformation of the branchial into the pulmonary circula- 

 tion is eflfected, and the heart assumes its new character of 

 a trilocular cavity ; possessing, that is to say, two auricles and 

 a single ventricle, by which the blood which is sent to the 

 lungs, and that which is distributed to the system at large, 

 is alike of a mixed character, as in the true Reptilia. 



The respiratory organs are no less surprisingly modified 

 during the progress of the changes just described in those of 

 circulation. The total loss of the branchiae, which are re- 



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