THE CIRCULATION" OF THE BLOOD. 177 



that on the right side of the heart. There is a slight vestige of the 

 foramen between the auricles, which exists in foetal life, on the septum 

 between them. 



Left Ventricle. Though taking part to a comparatively slight ex- 

 tent in the anterior surface, the left ventricle occupies the chief part of 

 the posterior surface. In it are two openings very close together, viz. 

 the auriculo-ventricular and the aortic, guarded by the valves corre- 

 sponding to those of the right side of the heart, viz. the bicuspid or 

 mitral and the semilunar or sigmoid. The first opening is at the left 

 and baek part of the base of the ventricle, and the aortic in front and 

 toward the right. In this ventricle, as in the right, are the columnae 

 carnese, which are smaller but more closely reticulated. They are chiefly 

 found near the apex and along the posterior wall. They will be again 

 referred to in the description of the valves. The walls of the left ven- 



Fig. 150. Network of muscular fibres from the heart of a pig. The nuclei of the muscle-corpus- 

 cles are well shown, x 450. (Klein and Noble Smith.) 



tricle, which are nearly half an inch in thickness, are, with the excep- 

 tion of the apex, twice or three times as thick as those of the right. 



Capacity of the Chambers. During life each ventricle is capable 

 of containing about four to six ounces (about 180 grms.) of blood. The- 

 capacity of the auricles after death is rather less than that of the ven- 

 tricles: 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 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 

 increasing 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; but a ring of connective tissue, to which 

 some of the muscular fibres are attached, is inserted between each auri- 

 cle and ventricle, and forms the boundary of the auriculo-ventricular 



12 



