182 CIRCULATION. 



the ventricle. When the organ is empty, these curtains are 

 applied to the walls of the ventricle, leaving the auriculo- 

 ventricular opening free ; but when the ventricle is com- 

 pletely filled, and the fibres contract, they are forced up, their 

 free edges become applied to each other, and the opening is 

 closed. 



Pulmonic Valves. These valves, also called the semi- 

 lunar or sigmoid valves of the right side, are situated at the 

 orifice of the pulmonary artery. They are strong membra- 

 nous pouches, with their convexities, when closed, looking 

 towards the ventricle. They are attached around the orifice 

 of the pulmonary artery, and are applied very nearly to the 

 walls of the vessel when the blood passes in from the ven- 

 tricle ; but at other times their free edges meet in the centre, 

 forming an effectual barrier to regurgitation. In the centre 

 of the free edge of each valve is a little corpuscle called the 

 corpuscle of Arantius / and just above these points of attach- 

 ment, the artery presents three little dilatations, or sinuses, 

 called the sinuses of Valsalva. The corpuscles of Arantius 

 have been supposed to facilitate the closure of the valves by 

 slightly removing them from the walls of the vessel, so that 

 the blood may get behind them. This, however, is probably 

 not their function. They aid in the adaptation of the valves 

 to each other, and the effectual closure of the orifice. 



Mitral Valve. This valve, sometimes called the bicuspid, 

 is situated at the left auriculo-ventricular orifice. It is called 

 mitral from its resemblance, when open, to a bishop's mitre. 

 It is attached to the edges of the opening, and its free borders 

 are held in place when closed by the chordae tendinese of the 

 left side. It presents no material difference from the tri- 

 cuspid valve, with the exception that it is divided into two 

 curtains instead of three. 



Aortic Valves. These valves, also called the semilunar 



