DOUBLE CIRCULATORY SYSTEM. 



377 



the lung-veins to the left auricle, and is driven through the 

 left ventricle into the body-arteries. Between the lung- 

 arteries and lung-veins is situated the capillary system of 

 the lesser, or lung-circulation ; between the body-arteries 

 and the body-veins lies the capillary system of the greater, 

 or body-circulation. Only in the two highest Vertebrate 



FIG. 300. The five arterial arches of Skulled Animals (1-5) in their 

 original form : a, arterial stalk ; a", main stem of the aorta ; c, head- 

 artery (carotis, anterior continuation of the aorta-roots). (After Rathke.) 



PIG. 301. The &ve arterial arches of Birds ; the light portions of the 

 rudiment disappear ; only the dark parts are permanent. Letters as in 

 Fig. 300 : s, arteries of the clavicula (sub-clavian) ; p, lung-artery j p' t 

 branches of the same. (After Rathke.) 



FIG. 302. The five arterial arches of Mammals. Letters as in Fig. 301 : 

 v, vertebral artery; b, Botalli's duct (open in the embryo, afterwards 

 closed). (After Rathke.) 



classes, in Birds and Mammals, is this complete separation 

 of the two courses of the circulation perfect. Moreover, this 

 separation has taken place in the two classes independently 

 of each other, as is shown by the unequal development of 

 the aortas. In Birds, which are descended from Reptiles, 



