VIII 



THE CIRCULATORY SYSTEM 



The relatively large heart (Fig. 12) is enclosed in 

 a thin, membranous pericardium, and lies partly 

 between the two lobes of the liver, partly cranial 

 to that organ. Conical in form, its apex is 

 directed in a caudal direction and slightly 

 to the left of the median plane. The exterior 

 shows a shallow groove indicating a division 

 into two atria, forming the base, and two 

 ventricles constituting the greater part of the 

 bulk of the organ. The walls of the atrial 

 portion are thin, while those of the ventricles 

 are much thicker. 



Into the right atrium open the large veins 

 — a caudal vena cava and two cranial venae 

 cavae — by which the blood is drained from all 

 parts of the body with the exception of the 

 lungs. A muscular fold, known as the 

 Eustachian valve, lies to the right of the 



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