V. THE CIRCULATORY SYSTEM 



The circulatory system is in reality made up of two systems, 

 the blood system and the lymphatic system. The blood system 

 is the one usually referred to In speaking of the circulatory sys- 

 tem, and includes the heart, blood, and blood vessels. The heart 

 acts as a muscular pump and forces blood through the vessels 

 to all parts of the body. The vessels carrying blood away from 

 the heart are called arteries, and those that bring the blood to the 

 heart are known as veins. The character of the blood carried 

 thus has nothing to do with the naming of the vessels. Arteries 

 and veins are connected by capillaries, thin-walled and micro- 

 scopic, through which interchange between the blood and the 

 tissues takes place. 



The lymphatic system consists of lymph vessels, sinuses, and 

 glands and conveys a colorless fluid, lymph, which bathes the 

 tissues of the body. Some of the lymph glands have already 

 been found. The lymph vessels are very delicate and no attempt 

 will be made to dissect them. 



Circulatory System of the Shark 



The arrangement of blood vessels in the elasmobranchs is 

 rather primitive and in many respects Is similar to embryonic 

 stages in the development of the mammalian circulation. 

 Hence, a comparison of the shark circulatory system with that 

 of the cat should present an Interesting study. 



External Anatomy of the Heart. — Make a median inci- 

 sion on the ventral side through the pectoral girdle and carefully 

 continue the incision forward to the mouth. The pericardial 

 cavity, bounded posteriorly by the transverse septtnn will be 

 exposed. Clean away the tissues ventral to this cavity and 

 expose the heart. The ventricle Is the most conspicuous portion 

 of the heart from a ventral view. Its walls are heavily muscular 

 and it contains a single cavity. The short, thick-walled vessel 

 leaving the ventricle Is the co7ius arteriosus. This continues 



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