212 THE CIRCULATORY SYSTEM [CH. XIX. 



lunar. The walls of the left ventricle, which in man are nearly 

 half an inch in thickness, are about three times as thick as those 

 of the right. 



Cavity of right ventricle. -L^ 



^M<V:- ^ssas. ^^^^. Cavity of left ventricle. 



FIG. 193. Transverse section of bullock's heart in a state of cadaveric rigidity. (Dalton.) 



Capacity of the Chambers. During life each ventricle is 

 capable of containing about three ounces of blood. ' The capacity of 

 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 that the ventricles are 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.), 3J inches (8 cm.) greatest width, and 2 

 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 main thickness of the heart-wall is composed 

 of muscular tissue; but a ring of connective tissue, to which many 

 of the muscular fibres are attached, lies between each auricle and 

 ventricle at the auriculo-ventricular orifice. The embryonic origin 

 of the heart from a single tube is indicated by the fact that the 

 superficial layers of muscle of the auricles and also of the ventricles 

 encircle the chambers of the two sides; this ensures their simul- 

 taneous contraction. Very varying accounts are given of the arrange- 

 ment of the layers of fibres especially in the ventricles, but the best 

 appears to be the following : in the interior we have muscular fibres 

 which are thrown into the columnse carnese, and papillary muscles ; 

 it may be called the papillary layer. Next come fibres arranged 

 circularly, some round the left, and others round the right ventricle ; 

 this layer called the circular layer comprises the main thickness of 

 the ventricular wall, and the act of propelling the blood into the 

 arteries is chiefly performed by it. External to this, arranged in three 

 principal bands, are fibres which spirally encircle one ventricle, and 



