THE HEART 175 



impulse against the chest wall being felt in the fifth 

 interspace on the left side, about 3| inches from the 

 middle of the sternum. The posterior or postero- 

 inferior surface is flat, formed chiefly by the wall of 

 the left ventricle, and rests on the diaphragm; its 

 anterior or anterosuperior surface is convex, formed 

 chiefly by the wall of the right ventricle, but also 

 partly by the left, and is overlapped by the lungs, 

 except in the central part. The borders of the heart 

 are right and left, the former is long and thin, the 

 left shorter and thicker. The length of the heart is 

 about 5 inches, the greatest breadth 3J inches; its 

 thickness about 2J inches. Its weight is approxi- 

 mately 10 to 12 ounces in the male, 8 to 10 ounces in 

 the female, and these increase with age. 



Externally its surface presents a deep transverse 

 groove, auriculoventricular, which marks an upper 

 auricular and lower ventricular portion; this latter 

 part presents a longitudinal furrow on the front and 

 the back, the former being somewhat to the left, the 

 latter to the right, marking off the right and left 

 ventricles (interventricular groove). 



The heart contains four chambers. They are divided 

 by a longitudinal partition or septum into a right and 

 left part, and these in turn are subdivided into an 

 auricle and a ventricle by a transverse partition, 

 which is perforated on each side by an opening, called 

 the auriculoventricular, connecting the auricle and 

 ventricle 



The Cavities of the Heart. The inner surface of 

 the heart muscle (myocardium) is lined throughout 

 by a thin layer of membrane called the endocardium. 

 It is continuous with the lining membrane of the 

 great bloodvessels opening into the heart cavities, 

 and helps to form the, various valves by a process,, 

 whereby the membrane is thrown into folds. The 

 endocardium consists of a flattened layer of endothelial 

 cells resting on a membrane formed of subendothelial 



