Parsons, Thomas S. Ramsay Wright Zoological Laboratories, University of Toronto, Ontario, Canada.
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The muscular pumping organ of the cardiovascular system of vertebrates. In vertebrate animals, the heart is a hollow muscular organ that functions to pump the blood through the circulatory system. It can contain up to four chambers (as is the case in mammals and birds, including humans; Fig. 1), and it works by rhythmically contracting and dilating. The heart typically lies ventrally, near the anterior end of the trunk; it is located ventral and medial to the gills in fish and at the base of the neck or in the chest region of tetrapods. In humans, it is located behind the breastbone and ribs between the third and fifth costal cartilages. Its anterior portion or base is directed to the right and dorsally and is the area where the great vessels enter and leave the heart. The lower muscular portion ends in a blunt apex that lies behind the fifth costal cartilage on the left. See also: Blood; Cardiovascular system; Muscle
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