CIRCULATION OF THE BLOOD. 



115 



Part of the blood thus forced back lies in the pouches (sinuses of Valsalva) 

 (a, Fig. 98, B) between the valves and the arterial walls; and the valves 

 are by it pressed together till their thin lunated margins meet in three 



FIG. 98. Sections of aorta, to show the action of the semilunar valves. A is intended to show 

 the valves, represented by the dotted lines, pressed toward the arterial walls, represented by the con- 

 tinuous outer line. B (after Hunter) shows the arterial wall distended into three pouches (a), and 

 drawn away from the valves, which are straightened into the form of an equilateral triangle, as rep- 

 resented by the dotted lines. 



lines radiating from the centre to the circumference of the artery (7 and 

 8, Fig. 99). 



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

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

 ders before mentioned. Among the cords which are interwoven in the 



FIG. 99. 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 au- 

 ricles have been removed immediately above the auriculo-ventricular orifices. The semilunar and 

 auriculo-ventricular valves are in the nearly closed condition. 1, 1, the base of the right ventricle; 

 1'. 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 Interior part of the pulmonary ar- 

 tery ; 8, placed upon the posterior part of the root of the aorta ; 9, the right, 9', the left coronary artery. 

 (Allen Thomson.) 



substance of the valves, 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. 



