210 THE CIKCULATORY SYSTEM [CH. XVI1J. 



smaller than that on the right side. There is a depression on the 

 septum between the auricles, which is a vestige of the foramen 

 between them, that exists in foetal life. 



The left ventricle occupies the chief part of the posterior surface. 

 In it are two openings very close together, viz., the auriculo-ventri- 

 cular and the aortic, guarded by the valves corresponding to those of 

 the right side of the heart, viz.. the 'bicuspid or mitral and the semi- 

 lunar. The first opening is at the left and back part of the base of 

 the ventricle, and the aortic in front and towards the right. In this 

 ventricle, as in the right, are the columnae carneae, which are smaller 

 but more closely reticulated. They are chiefly found near the apex 

 and along the posterior wall. 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 



Cavity of right ventricle. _ _ . 



FIG. 219. Transverse section of bullock's heart in a state of calaveric rigidity. (Ualton.) 



the auricles is rather less than that of the ventricles: the thick- 

 ness 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 2J 

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

 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 (figs. 113 and 220); but a ring 

 of connective tissue, to which some of the muscular fibres are 

 attached, is inserted between each auricle and ventricle, and forms 

 tin boundary of the auricula -ventricular opening. Fibrous tissue also 

 exists at the origins of the pulmonary artery and aorta. 



