130 HUMAN PHYSIOLOGY. 



tiring about 5J4 inches in length and about 3^ in breadth, weighing 

 from 10 to 12 ounces in the male and from 8 to 10 in the female. 

 Situated in the thoracic cavity, between the lungs, its base is directed 

 upward, backward, and to the right ; its apex is directed downward 

 and to the left. 



Pericardium. The heart is surrounded by a closed fibrous mem- 

 brane called the pericardium. The inner surface of this membrane 

 is lined by a serous membrane, which is also reflected over the 

 surface of the heart ; between the two surfaces of the serous mem- 

 brane is found a small quantity of fluid (the pericardial fluid), which 

 lubricates the surfaces and prevents friction during the movements 

 of the heart. The interior of the heart is also lined by a serous 

 membrane, called the endocardium. 



Cavities of the Heart. The general cavity of the heart is sub- 

 divided by a longitudinal septum into a right and left half; each of 

 these cavities is in turn subdivided by a transverse constriction into 

 two smaller cavities, which communicate with each other and are 

 known as the auricles and ventricles, the orifice between the auricle 

 and ventricle being known as the auriculoventricular orifice. The 

 heart, therefore, consists of four cavities a right auricle and ven- 

 tricle and a left auricle and ventricle. 



Into the right auricle the two terminal trunks of the venous sys- 

 tem the superior and inferior vena caves empty the venous blood 

 which has been collected from all parts of the system ; from the right 

 ventricle arises the pulmonary artery, which, passing into the lungs, 

 distributes the blood to the walls of the air-cells of the lungs ; into 

 the left auricle empty four pulmonary veins, which have collected 

 the blood from the lung capillaries ; from the left ventricle springs 

 the aorta, the general trunk of the arterial system, the branches of 

 which distribute the blood to the entire system. 



The Valves of the Heart. The valves of the heart are formed 

 by a reduplication of the endocardium strengthened by connective 

 tissue. At the auriculoventricular openings on the right and left 

 sides of the heart, respectively, are found the ' tricuspid and mitral 

 valves. The tricuspid valve consists of three, the mitral of two, 

 cusps or segments, which project into the interior of the ventricle 

 when it does not contain blood. At their bases the segments are 

 united so as to form an annular membrane attached to the margin 

 of the orifice. To the free edges of the valves are attached numer- 



