THE CIRCULATION. 225 



the artery. Behind each valve the artery is dilated into a shallow 

 pouch, called the sinus of Valsalva, which prevents the valve, when 

 open, from adhering to the side of the artery and permits the reflow 

 of blood to readily press the valve down to close the opening. At 

 the middle of the free border of the valve there is a thickening of 

 fibrous tissue, making the corpora Arantii. The left ventricle is 

 three times the thickness of the right, and its apex forms the apex 

 of the heart. It is longer and forms more of the posterior surface 

 of the heart than the right ventricle. Like the right ventricle, it 

 has columns earner, papillary muscles, and chordae tendineae. 



Pulmonary valve. 



i 



Bicuspid 

 valve. "" 



Aortic 

 valve. 



f Tricuspid 

 '*' valve. 



Fig. 65. Valves of Heart. 



The left auriculo-ventricular valve is provided with a pair of 

 membranous folds forming the mitral valve, or bicuspid valve. It 

 is larger in size and thicker than the right auriculo-ventricular 

 valve. These mitral segments have the chords tendinese attached. 



The left ventricle has an opening which is the origin of the 

 great blood-vessel, the aorta. It is provided with semi-lunar or 

 sigmoid valves, of the same character as those of the pulmonary 

 artery. 



Structure of the Heart. 



The lining membrane of the heart is called the endocardium. 

 All the valves of the heart are made up by its inclosing fibrous 

 tissue. The endocardium is formed of epithelium and fibre-elastic 



