FUNCTION OF THE VALVES. 127 



soon as it relaxes, and the dilated arterial walls can recoil 

 by their elasticity, they press the blood as well towards the 

 ventricles as onwards in the course of the circulation. Part 

 of the blood thus pressed back lies in the pouches (a, fig. 

 37, 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. 38). 

 Fig. 38.* 



* Fig. 38. View of the base of the ventricular part of the heart, 

 showing the relative position of the arterial and auriculo- ventricular 

 orifices. . The muscular fibres of the ventricles are exposed by the 

 removal of the pericardium, fat, blood-vessels, etc. ; the pulmonary 

 artery and aorta have been removed by a section made immediately 

 beyond the attachment of the semilunar valves, and the auricles have 

 been removed immediately above the auriculo -ventricular orifices. The 

 semilunar and auriculo-ventricular valves are in the nearly closed con- 

 dition, i, i, the base of the rigbt ventricle ; i', the conus arteriosus ; 

 2, 2, the base of the left ventricle ; 3, 3, the divided wall of the right 

 auricle; 4, that of the left; 5, 5', 5", the tricuspid valve; 6, 6', the 

 mitral valve. In the angles between these segments are seen the 

 smaller fringes frequently observed ; 7, the anterior part of the pul- 

 monary artery ; 8, placed upon the posterior part of the root of the 

 aorta ; 9, the right, 9', the left coronary artery. (From Quain's 

 Anatomy). 



