96 ZOOLOGY SECT. 



directly : there it enters the left side of the auricle, in which 

 a vertical partition is developed, separating a left auricle (/I 1 ), 

 which receives the aerated blood from the lungs, from a right 

 auricle (A), into which is poured the impure blood of the sinus 

 venosus. Lastly, in Crocodiles, Birds, and Mammals (B) the 

 ventricle also becomes divided into right and left chambers, and we 

 get a four-chambered heart, having right and left auricles and right 

 and left ventricles : at the same time the conus arteriosus and sinus 

 venosus cease to exist as distinct chambers. The left auricle receives 

 aerated blood from the lungs and passes it into the left ventricle, 

 whence it is propelled through the system : the right auricle receives 

 impure blood from the system, and passes it into the right ventricle 

 to be pumped into the lungs for aeration. Thus the four-chambered 

 heart of the higher Vertebrata is quite a different thing from that 



V 



FIG. 775. Diagram of the heart A, in an Amphibian ; B, in a Crocodile. A, right auricle ; 

 A', left auricle ; Aji, pulmonary artery ; Ir, pulmonary vein ; RA, aortic arches ; V. ventricle ; 

 V, left ventricle ; v, v, and Ve, Ve, pre- and postcavals. (From Wiedersheim's Vertebrata.) 



of a Fish : in the latter the four chambers sinus venosus, auricle, 

 ventricle, and conus arteriosus form a single longitudinal series, 

 whereas in a Mammal, for instance, the four chambers constitute 

 practically a double heart, there being no direct communication 

 between the auricle and ventricle of the right side, or respiratory 

 heart, and those of the left side, or systemic heart. The modifica- 

 tions undergone by the arteries and veins in the higher Vertebrata 

 will be best considered under the various classes. 



It will be noticed that there is a sort of rough correspondence 

 between the blood-vessels of Craniata and those of the higher 

 Worms. The sub-intestinal vein, heart, and ventral aorta together 

 form a ventral vessel, the dorsal aorta a dorsal vessel, and the aortic 

 arches transverse or commissural vessels. The heart might thus be 

 looked upon as a portion of an original ventral vessel, which has 

 acquired strongly muscular walls, and performs the whole function 

 of propelling the blood. But in making such a comparison it 



