120 HANDBOOK OF PHYSIOLOGY. 



This position of the valves and arterial walls is retained so long as the 

 ventricle continues in contraction : but, as soon as it relaxes, and the 

 dilated arterial walls can recoil by their elasticity, the blood is forced 

 backwards towards the ventricles as onwards in the course of the circu- 

 lation. Part of the blood thus forced back lies in the pouches (sinuses 

 of Valsalva) (a, Fig. 112, B) between the valves and the arterial walls ; 

 and the valves are by it pressed together till their thin lunated margins 

 meet in three lines radiating from the centre to the circumference of the 

 artery (7 and 8, Fig. 113). 



The contact of the valves in this position,' and the complete closure 

 of the arterial orifice, are secured by the peculiar construction of their 

 borders before mentioned. Among the cords which are interwoven in 



the substance of the valve, are two of greater 

 strength and prominence than the rest ; of 

 which one extends along the free border of 

 each valve, and the other forms a double 

 curve or festoon just below the free border. 

 Each of these cords is attached by its outer 

 extremities to the outer end of the free margin 

 of its valve, and in the middle to the corpus 

 Arantii; they thus inclose a lunated space 

 ,_ j from a line to a line and a half in width, in 



which space the substance of the valve is 

 much thinner and more pliant than elsewhere. 

 FIG. 114. -vertical section When the valves are pressed down, all these 



through the aorta at its junction r . 



with the left ventricle, a, Sec- parts or spaces of their surfaces come into 



tion of aorta. 66, Section of two r 



valves, c, section of wall of ven- contact, and the closure of the arterial orifice 



tricle. d, Internal surface of 



ventricle. is thus secured by the apposition not of the mere 



edges of the valves, but of all those thin lunated parts of each which lie 

 between the free edges and the cords next below them. These parts are 

 firmly pressed together, and the greater the pressure that falls on them 

 the closer and more secure is their apposition. The corpora Arantii 

 meet at the centre of the arterial orifice when the valves are down, and 

 they probably assist in the closure ; but they are not essential to it, for, 

 not unfrequently, they are wanting in the valves of the pulmonary 

 artery, which are then extended in larger, thin, flapping margins. In 

 valves of this form, also, the inlaid cords are less distinct than in those 

 with corpora Arantii ; yet the closure by contact of their surfaces is not 

 less secure. 



It has been clearly shown that this pressure of the blood is not en- 

 tirely sustained by the valves alone, but in part by the muscular substance 

 of the ventricle (Savory). By making vertical sections (Fig. 114) 

 through various parts of the tendinous rings it is possible to show clearly 

 that the aorta and pulmonary artery, expanding towards their termina- 



