Metazoa Rana. 291 



these viscera with nutriment and oxygen. The purest blood 

 passes by the carotids to the head region, where naturally 

 the purest blood is wanted. 



Similarly the impure blood is returned to the heart by 

 special veins from each organ, uniting ultimately into four 

 large vessels, two anterior or precaval veins and one posterior 

 or postcaval vein which run direct to the sinus venosus, 

 and one anterior abdominal, which, after traversing the 

 ventral body-wall goes to the liver. 



The details of the arrangement and distribution of the 

 smaller branches though of great importance cannot be dealt 

 with here. Figs. 148, 153, and 154 will, however, show 

 many of these details, which can be made out without diffi- 

 culty in the animal itself. (The names of the various veins 

 and arteries indicate sufficiently the origin and distribution.-) 

 One point must, however, be briefly noticed with regard to 

 the renal circulation. The common aorta gives off a number 

 of branches to each of the two kidneys lying in the dorsal 

 portion of the ccelom, while the renal veins open into the 

 postcaval : this is the normal arrangement of artery and 

 vein. But in the frog an additional means of circulation is 

 present in the shape of a large vessel known as the renal- 

 portal, which carries venous blood from the hind-limb and 

 lumbar region to the kidney. The renal-portal on either 

 side gives off a branch to the anterior abdominal, so that 

 blood coming from the hind-limbs may pass to the heart 

 by two channels, through the liver by the anterior abdomi- 

 nal, or through the kidney by the renal-portal. 



Respiratory system, Before venous blood can be made 

 use of again as a carrier of oxygen to the tissues it must be 

 purified by the removal from it of the carbonic acid, water, 

 and other waste products collected in its passage through 

 the systemic capillaries. We shall consider first the re- 

 moval of the carbonic acid. This duty is performed by the 

 lungs, two semi-transparent sacs of large size situated 

 ventrally to the oesophagus. At the posterior end of the 



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