SECTION V. THE VASCULAR SYSTEM 



The circulatory or blood-vascular system of the frog is a 

 closed system, the blood being constantly confined within 

 vessels. A heart, divided into two atria and one ventricle, 

 pumps the blood through these vessels. From the ventricle 

 the blood passes into one of three pairs of arches. The 

 first, or carotid arches, carry blood to the head; the second, 

 or systemic arches, carry blood to the rest of the body, 

 uniting to form the dorsal aorta; the third, or pulmocutane- 

 ous arches, carry blood to the lungs and skin to be aerated. 

 All the arteries terminate in tiny capillaries which unite 

 to form veins. From the head and forelimbs the blood is 

 returned by the precaval veins to the right atrium ; from the 

 viscera to the hepatic portal vein, terminating in the liver; 

 from the hindlegs to the hepatic portal vein by the ab- 

 dominal vein, or to the kidney by the renoportal veins; 

 from the liver and the kidney to the right atrium by the 

 postcaval vein; and from the lungs by the pulmonary veins 

 to the left atrium. 



You have already identified the' coeliaco-mesentenc artery 

 jrom the dorsal aorta to the viscera, and the hepatoportal 

 veins (Drawings 5 and 6). 



1. The Heart 



It is desirable at this point to make use of a fresh specimen, 

 preferably injected. Open along the mid-ventral line as 

 before. Remove the sternum (235) so as to expose the 

 pericardial cavity (29) completely. Raising the pericar- 



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