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THE HEART 2
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In mammals, the sinus venosus
is absorbed into the wall of the right atrium and the blood vessels which
had emptied into the sinus (the superior and inferior vena cava and the
coronary sinus) empty into the right atrium instead.
The embryonic sinus venosus is present
in adult mammals only as the SA node (Kardong,461, 475). The
valves of the sinus venosus become the valve
of the inferior vena cava and the valve of the coronary sinus in the human
fetal heart (Torrey, 1979, p. 429-30). Edentates retain vestiges of the sinus valves
and a thin sinus venarum, a nonfunctional vestige
of the sinus venosus (Webster, 1974, p. 284). Mammalian hearts also possess an AV node in
their conduction system (Kardong). Mammalian cardiac muscle possesses T-tubules
not present in the cardiac muscle cells of reptiles and amphibians (Webster,
1974, p. 179). While birds and mammals may share traits
such as a four chambered heart, these traits arose independently in evolution.
In mammals, the interventricular septum is formed by the fusing of muscular
ridges and their endocardia while in birds it
is formed by the fusion of muscular trabeculae
with little input from the endocardium. The partial interventricular
septum in lungfish is actually formed by a combination of these two mechanisms.
Mammals and birds also differ the manner in which interatrial
openings close at birth. In addition,
birds still retain a portion of the sinus venosus
separated from the right atrium by a sulcus
in addition to the internal valves of the sinus venosus
(Torrey, 1979). In placental mammals, the ductus arteriosus is functional
in the embryo but becomes the ligamentum arteriosum in adults. (Kardong 479)
A conus arteriosus exists in edentates, dogs, porpoises, and bats.
Bats retain a muscular conus
which contracts as a separate chamber (Webster, 1974). Some mammals have a septal
artery to service the interventricular septum
(Weichert, 1970). |
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OPOSSUM |
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CAT |
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GOAT |
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COW |
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SHEEP |
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PIG |
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MONKEY HEART |
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HUMAN HEART MODEL |
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