438 



HEALTH AND DISEASE 



it has been named the systemic heart, and is the main agent in effecting 

 the greater or systemic circulation. The right heart forces dark or venous 

 blood, charged with carbon dioxide, through the lungs, where, becoming 

 aerated by losing that gas and absorbing oxygen, it returns to the left 

 heart as bright or arterial blood, and is distributed to the body generally. 

 The heart as a whole weighs about 7 Ibs. Its length is about 10 inches, 

 and its width at the base about 7 or 8 inches. It is of conical form, 

 the base being directed to the spine. The capacity of each cavity is 

 estimated to be about 1|- pint. The heart is composed of a form of 

 muscular tissue intermediate between the striated and unstriated. The 



Figs. 189 and 190. Diagrammatic Views of the Heart, showing Valves open and closed 



A, Pulmonary Artery. B, Posterior Vena Cava. C, Anterior Vena Cava. R.A., Right Auricle. * Auriculo- 

 Ventricular Opening (open in the first figure and closed in the second). R.V., Right Ventricle. -9 > Course 

 of Blood. s.v., Semilunar Valves (closed in the first figure, open in the second). 



external or superficial layers run obliquely over both ventricles; the inner 

 or deeper layers are confined to the right and left ventricles respectively. 

 Each heart presents two cavities, an upper one, named the auricle, and 

 a lower one, named the ventricle; so that there are two auricles and two 

 ventricles, or four cavities in all. The right auricle receives the two 

 venae cavse or large systemic veins above, and opens by a wide orifice 

 into the right ventricle below, the aperture being guarded by the 

 tricuspid valve. From the upper part of the right ventricle the pul- 

 monary artery arises, the orifice of which is also guarded by the three 

 semilunar valves. A similar arrangement exists on the left side. The 

 pulmonary veins have no valves. They open into the left auricle above, 

 and this communicates by a large orifice, guarded by the bicuspid valve, 

 with the left ventricle below. From the left ventricle arises the aorta, 



