2O8 THE CIRCULATORY SYSTEM. [CH. xvii]. 



along the posterior wall. They will be again referred to with the 

 description of the valves. The walls of the left ventricle, which 

 are nearly half an inch in thickness, are, with the exception of the 

 apex, about three times as thick as those of the right. 



Capacity of the Chambers. During life each ventricle is 

 capable of containing about three ounces of blood. The capacity 

 of the auricles after death is rather less than that of the 

 ventricles : the thickness of their walls is considerably less. 

 The latter condition is adapted to the small amount of force 

 which the auricles require in order to empty themselves into 

 their adjoining ventricles ; the former to the circumstance of the 

 ventricles being partly filled with blood before the auricles contract. 



Size and Weight of the Heart. The heart is about 5 inches 

 long (about 12 '6 cm.), 3^ inches (8 cm.) greatest width, and 2\ 

 inches (6-3 cm.) in its extreme thickness. The average weight of 



Fig. 211. Network of muscular fibres from the heart of a pig. The nuclei are well shown. 

 X 450. (Klein and Noble Smith.) 



the heart in the adult is from 9 to 10 ounces (about 300 grms.) ; 

 its weight gradually increases throughout life till middle age ; 

 it diminishes in old age. 



Structure. The walls of the heart are constructed almost 

 entirely of layers of muscular fibres (p. 94) ; but a ring of con- 

 nective tissue, to which some of the muscular fibres are attached, 

 is inserted between each auricle and ventricle, and forms the 

 boundary of the auricula-ventricular opening. Fibrous tissue also 

 exists at the origins of the pulmonary artery and aorta. 



The muscular fibres of each auricle are in part continuous with 

 those of the other, and partly separate; and the same remark 

 holds true for the ventricles. Some muscular fibres also pass 

 across the tendinous ring which separates each auricle from the 

 corresponding ventricle. 



Endocardium. As the heart is clothed on the outside by the 

 epicardium, so its cavities are lined by a smooth and shining 



