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VALVES AND VESSELS

     The presence of valves in the cardiovascular and lymphatic systems prevents the inappropriate flow of blood and lymph.   In humans, there are 4 valves in the heart (2 atrioventricular valves and two semilunar valves) and numerous valves in veins and lymphatic vessels.   Venous valves are known from Amphioxus and valves may also exist in arteries in Amphioxus and sharks (Dutta, 367; Webster, 1974, p. 67; Hoar, 1983).

     The primitive heart of urochordates lacks valves, allowing the flow of blood to periodically reverse (Harris).  In hagfish, blood flows in one direction only and there are sinoatrial, atrioventricular, and semilunar heart valves.  (Kardong, 463).  The lamprey heart also possesses these valves and chordae tendinae attach to the atrioventricular valve. (Weichert, 1970).  Although this basic pattern of heart valves was established early, minor variations of the valves and their chordae tendinae are occasionally found in higher vertebrates.  For example, in monotremes, the left AV valve (the “bicuspid”) possesses three cusps (Weichert, 1970).   In marsupials and placentals, an occasional chordae tendinae is muscular (Weichert, 1970, p. 556).

     The hearts of more primitive vertebrates possess a chamber known as the conus (or truncus) arteriosus, which is lacking in adult humans.  In many fish, the conus arteriosus contains one or several semilunar valves.  Lampreys have two such valves.  Cartilaginous and ganoid fish have the highest number of valves in the truncus arteriosus; Epiceratodus has 8 sets of valves (Weichert, 1970).

    Most vertebrates possess a heart chamber known as the sinus venosus which empties into the right atrium.  In most fish (with lungfish being an exception) a valve exists between sinus venosus and the atrium (Webster, 1974).  Bats and moles possess a semilunar valve at the entrance to the right atrium, other mammals possess a structure called the valve of Eustachius (Weichert, 1970).

SHARK

SHARK HEART

SHARK HEART
SHARK HEART  

GAR

GAR HEART

OPOSSUM

OPOSSUM HEART

OPOSSUM HEART
OPOSSUM HEART
OPOSSUM HEART
OPOSSUM HEART
OPOSSUM HEART
OPOSSUM HEART OPOSSUM HEART
OPOSSUM HEART OPOSSUM HEART
OPOSSUM HEART OPOSSUM HEART