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| 61. | interatrial septum | cardiac septum located between the two atria; contains the fossa ovalis after birth |
| 62. | intercalated disc | physical junction between adjacent cardiac muscle cells; consisting of desmosomes, specialized linking proteoglycans, and gap junctions that allow passage of ions between the two cells |
| 63. | internodal pathways | specialized conductile cells within the atria that transmit the impulse from the SA node throughout the myocardial cells of the atrium and to the AV node |
| 64. | interventricular septum | cardiac septum located between the two ventricles |
| 65. | isovolumic contraction | (also, isovolumetric contraction) initial phase of ventricular contraction in which tension and pressure in the ventricle increase, but no blood is pumped or ejected from the heart |
| 66. | isovolumic ventricular relaxation phase | initial phase of the ventricular diastole when pressure in the ventricles drops below pressure in the two major arteries, the pulmonary trunk, and the aorta, and blood attempts to flow back into the ventricles, producing the dicrotic notch of the ECG and closing the two semilunar valves |
| 67. | left atrioventricular valve | (also, mitral valve or bicuspid valve) valve located between the left atrium and ventricle; consists of two flaps of tissue |
| 68. | marginal arteries | branches of the right coronary artery that supply blood to the superficial portions of the right ventricle |
| 69. | mesoderm | one of the three primary germ layers that differentiate early in embryonic development |
| 70. | mesothelium | simple squamous epithelial portion of serous membranes, such as the superficial portion of the epicardium (the visceral pericardium) and the deepest portion of the pericardium (the parietal pericardium) |
| 71. | middle cardiac vein | vessel that parallels and drains the areas supplied by the posterior interventricular artery; drains into the great cardiac vein |
| 72. | mitral valve | (also, left atrioventricular valve or bicuspid valve) valve located between the left atrium and ventricle; consists of two flaps of tissue |
| 73. | moderator band | band of myocardium covered by endocardium that arises from the inferior portion of the interventricular septum in the right ventricle and crosses to the anterior papillary muscle; contains conductile fibers that carry electrical signals followed by contraction of the heart |
| 74. | murmur | unusual heart sound detected by auscultation; typically related to septal or valve defects |
| 75. | myocardial conducting cells | specialized cells that transmit electrical impulses throughout the heart and trigger contraction by the myocardial contractile cells |
| 76. | myocardial contractile cells | bulk of the cardiac muscle cells in the atria and ventricles that conduct impulses and contract to propel blood |
| 77. | myocardium | thickest layer of the heart composed of cardiac muscle cells built upon a framework of primarily collagenous fibers and blood vessels that supply it and the nervous fibers that help to regulate it |
| 78. | negative inotropic factors | factors that negatively impact or lower heart contractility |
| 79. | P wave | component of the electrocardiogram that represents the depolarization of the atria |
| 80. | Purkinje fibers | specialized myocardial conduction fibers that arise from the bundle branches and spread the impulse to the myocardial contraction fibers of the ventricles |
| 81. | pacemaker | cluster of specialized myocardial cells known as the SA node that initiates the sinus rhythm |
| 82. | papillary muscle | extension of the myocardium in the ventricles to which the chordae tendineae attach |
| 83. | pectinate muscles | muscular ridges seen on the anterior surface of the right atrium |
| 84. | pericardial cavity | cavity surrounding the heart filled with a lubricating serous fluid that reduces friction as the heart contracts |
| 85. | pericardial sac | (also, pericardium) membrane that separates the heart from other mediastinal structures; consists of two distinct, fused sublayers: the fibrous pericardium and the parietal pericardium |
| 86. | pericardium | (also, pericardial sac) membrane that separates the heart from other mediastinal structures; consists of two distinct, fused sublayers: the fibrous pericardium and the parietal pericardium |
| 87. | positive inotropic factors | factors that positively impact or increase heart contractility |
| 88. | posterior cardiac vein | vessel that parallels and drains the areas supplied by the marginal artery branch of the circumflex artery; drains into the great cardiac vein |
| 89. | posterior interventricular artery | (also, posterior descending artery) branch of the right coronary artery that runs along the posterior portion of the interventricular sulcus toward the apex of the heart and gives rise to branches that supply the interventricular septum and portions of both ventricles |
| 90. | posterior interventricular sulcus | sulcus located between the left and right ventricles on the anterior surface of the heart |
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