THE CIRCULATION OF THE BLOOD. 



173 



The heart is situated in the chest behind the sternum and costal 

 cartilages, being placed obliquely from right to left, quite two-thirds of 

 it being to the left of the mid-sternal line. It is of pyramidal shape, 

 with the apex pointing downward, outward, and toward the left, and the 

 base backward, inward, and toward the right. It rests upon the dia- 

 phragm, and its pointed apex, formed exclusively of the left side of the 

 heart, is in contact with the chest wall, and during life beats against it 

 at a point called the apex beat, situated in the fifth left intercostal 

 space, and about three inches from the mid-sternal line. The heart is, 

 as it were, suspended in the chest by the large vessels which proceed 

 from its base, but, excepting at this part, the organ itself lies free within 

 the sac of the pericardium. The part which rests upon the diaphragm 



Right lung 



Pulmonary artery 



Left lung 



Diaphragm 



Fig. 146. View of heart and lungs in situ. The front portion of the chest-wall, and the outer 

 or parietal layers of the pleuree and pericardium have been removed. The lungs are partly col- 

 lapsed. 



is flattened, and is known as the posterior surface, while the free upper 

 part is called the anterior surface. The margin toward the left is thick 

 and obtuse, while the lower margin toward the right is thin and acute. 



On examination of the external surface the division of the heart into 

 parts which correspond to the chambers inside of it may be traced, for 

 a deep transverse groove called the auriculo-ventricular groove divides 

 the auricles which form the base of the heart from the ventricles which 

 form the remainder, including the apex, the ventricular portion being 

 by far the greater; and, again, the inter-ventricular groove runs between 

 the ventricles both front and back, and separates the one from the other. 

 The anterior groove is nearer the left margin and the posterior nearer 

 the right, as the front surface of the heart is made up chiefly of the 

 right ventricle and the posterior surface of the left ventricle. In the 



