THE CIRCULATING SYSTEM. 



with three corresponding vessels going to the dorsal aorta, 

 on each side direct communications taking place between 



FIG. 359. THE CIRCULATION OF A TADPOLE in its primitive stage, when nearly 

 all the blood is distributed to the gills ; the pulmonary arteries being quite 

 rudimentary, and the vessel (or ductus Botalli) connecting together the bran- 

 chial artery and vein at the root of each gill, being minute. 



a, bulbus aortae ; ba, branchial arteries; br l ,br 3 , and br^>, the three gills (or 

 branchiae of each side); bv, the branchial veins which bring back the blood 

 from the gills the hindmost pair of branchial veins on each side unite to form 

 an aortic arch (aa^, which again unites with its fellow of the opposite side to 

 form da, the descending (or dorsal) aorta. The branchial veins of the foremost 

 gills gives rise to the carotid arteries, cc. o, artery going to the orbit ; fa, 

 pulmonary artery :. i, 2, 3, anastomosing branches connecting together the 

 adjacent branchial arteries and veins. 



neighbouring arteries and veins (by what is called a ductus 

 Botalli\ in spite of each artery and. vein minutely dividing 

 in the gill beyond such points of communication. 



b,- 3 



FIG. 360. THE CIRCULATION IN A TADPOLE 



AT A MORE ADVANCED STAGE, when the 



gills have begun to be absorbed, the pulmo- 

 nary arteries to increase, as also the con- 

 necting branches (at the root of the gills) 

 between the branchial arteries and branchial 

 veins. 



The letters refer to the same parts as 

 > n Fig. 359. 



FIG. 361. THE CIRCULATION 

 IN A YOUNG FROG. Here the 

 gills have been absorbed, and 

 the blood passes directly from 

 the heart to the head, the 

 dorsal aorta, the lungs, and 

 the skin. 



The letters refer to the same 

 parts as in Fig. 359. 



Again, we may find, as in the adult Frog, three aortic arches 

 on each side, whereof the first is the common carotid, the 

 E E 



