THE HEART. 191 



being formed almost exclusively by the continuation of the 

 fibres of the left ventricle. Where the large columnae carneae 

 elevate themselves on its surface, its thickness is increased^ 

 Its shape is somewhat triangular. It forms a round projection 

 into the right ventricle, while its other surface, which presents 

 to the left, is concave to the same degree. It is rather thinner 

 as it approaches the auricular septum than elsewhere. Its 

 fibres near the apex are less closely connected to each other. 



The orifice of the Aorta is furnished with three semi-lunar 

 Valves,* which, in the mode of their arrangement, correspond 

 precisely with those of the pulmonary artery. They are, how- 

 ever, thicker, and the Corpuscula Aurantii are larger. The 

 Sinuses of Valsalva, attended with a slight dilatation of the ar- 

 tery, exist in the same way. Just beyond the margins of the 

 right and of the left valves are observed the orifices of the two 

 coronary arteries. The orifice of the aorta is somewhat ten- 

 dinous, which marks out the distinction of structure between 

 it and the ventricle. 



Of the Texture of the Heart. 



The Heart, with the exception of the serous membrane which 

 lines its cavities, and of the serous lamina of pericardium which 

 covers its surface, consists entirely of muscular fibres. 



The sides of the auricles, as stated, are much thinner than 

 those of the ventricles. In the right auricle, the stratum of 



* Mr. Erskine Hazard has furnished me with the following 1 estimate on the 

 action of these valves : 



If the diameter of the artery be put= 10, the 

 length of the superior edge of the valve will also 

 be 10. The arc occupied by the valve will be 

 10.47 = 120 of the circle. The valves, when 

 open, will either assume the form at B, or that of 

 the double chord of 60, as at A. In either case, 

 being .47 shorter than the arc, they cannot come 

 in contact with it, and must, therefore, leave room 

 for the blood to get behind them, and depress 

 the valves. For the same reason, they cannot 

 close the orifice of the coronary arteries. The 

 chord of 120 would b: 8.67. 



