294 THE VASCULAR SYSTEM 



The Left Ventricle pumps the blood into the ascending 

 aorta, and its walls, which possess trabeculse carnese, papillary 

 muscles, etc., are very similar to those of the right ventricle, 

 except that they are nearly three times as thick. 



The orifices of the pulmonary artery and the aorta are 

 guarded by endocardial valves, which consist of three 

 semilunar cusps. During ventricular systole, the cusps are 

 pressed apart and separated, but during diastole they are 

 thrust together across the orifice and so prevent regurgitation 

 into the ventricles. 



Surface Relations of the Heart. In mapping out the 

 antero - superior surface of the heart, the identification of 

 the individual ribs is essential. The union between the 

 manubrium and the body of the sternum is marked by a 

 transverse ridge, which can readily be felt through the skin. 

 At the extremities of this ridge, which is termed the sternal 

 angle (of Louis), the second costal cartilages articulate with 

 the sternum and they can be identified, therefore, in every 

 case. The third, fourth and fifth costal cartilages can be 

 distinguished without difficulty, but it is not easy to identify 

 the succeeding cartilages, as the spaces between them are 

 much narrower. 



The right border of the heart can be mapped out by a line 

 which commences above on the right third cartilage, and 

 descends with a slight convexity to the right. Opposite the 

 fourth intercostal space this line reaches its maximum distance 

 about 1 1 inches from the median plane, and it terminates 

 below on the sixth costal cartilage i inch from the median 

 plane. This border is formed by the right atrium (auricle) 

 alone. 



The inferior border of the heart can be represented by a 

 line which begins at the lower extremity of the right border 

 and passes to the left to reach the position of the apex-beat. 

 Under normal conditions the apex of the heart lies in the fifth 

 left intercostal space at a distance of 3^ inches from the 

 median plane. The lower border of the heart is formed, for 



