CH. XVIII.] 



VALVES OF THE HEART. 



211 



the free edge, on each side of the corpus Arantii. Here the valve 

 is thinnest, and composed of little more than the endocardium. 

 Thus constructed and attached, the three semilunar pouches are 

 placed side by side around the arterial orifice of each ventricle ; 

 they are separated by the blood passing out of the ventricle, but 

 immediately afterwards are pressed together so as to prevent any 

 return (6, fig. 208, and 7, fig. 209). Opposite each of the 

 semilunar cusps, both in the aorta and pulmonary artery, there 



Pulmonary 

 artery. 



Superior eava or 

 vein from head 

 and neck. 



Right auricle. 

 Inferior vena 

 cava. 



Right ventricle. 



Pulmonary veins 

 Aorta. 



Arteries to head 

 and neck. 



Left ventricle. 



Portal circula- 

 tion. 



Gastric and in- 

 testinal vessels. 



First renal cir- 

 culation. 



Systemic 



capillaries. 



Fig. 213. Diagram of the circulation. 



is a bulging outwards of the wall of the vessel : these bulgings 

 arc called the sinuses of Valnalva. 



Structure. The valves of the heart are formed of a layer of 

 closely woven connective and elastic tissue, over which, on every 

 part, the endocardium is reflected. 



Course of the Circulation. 



The blood is conveyed away from the left ventricle (as in the 

 diagram, fig. 213) by the aorta to the arteries, and returned to 

 the right auricle by the reins, the arteries and veins being 



P 2 



