HEART 



179 



aorta contain neither muscle fibers nor vessels. Their elastic fibers are 

 found chiefly on the ventricular sides of the valve, and in the noduli 

 (which are thickenings in the middle of the circumference of each segment, 

 to perfect their approximation when closed). 



Myocardium. The myocardium consists of muscle fibers arranged 

 in layers or sheets, which are wound about the ventricles in complex spirals, 

 making a vortex at the apex of each ventricle. If the heart is boiled in 

 dilute acid these layers may be unwound, and the heart has frequently 

 been investigated in this way, most recently by Mall (Amer. Journ. 

 Anat., 1911, vol. n, pp. 211-266). The layers are composed of cardiac 

 muscle, which is a syncytium of striated fibers with central nuclei and 



Myocardium. 



Endocardium 



Endothelium. - 



Detached 

 endothelial cell 



Small artery. 



Fibers of the atrio- 

 ventricular system. 



Connective tissue. 



Nucleus of a con- 

 nective tissue cell. 



Nucleus of 

 cardiac muscle. 

 Sarcoplasm. 



Capillaries. 



FIG. 172. FROM A CROSS SECTION OK THE PECTINATE MUSCLES OF A HUMAN HEART (RIGHT ATRIUM). 



X 240. 

 The muscle fibrils in transverse sections appear as points; at x they are radially arranged. 



intercalated discs, as already described (p. 129). Cardiac muscle is shown 

 in longitudinal section in Fig. 121 (p. 129), and in transverse section in 

 Fig. 172. Between the muscle fibers there are capillary branches of the 

 coronary vessels which ramify in the epicardium. The capillaries come 

 into close relation with the muscle fibers and some of them extend into 

 the endocardium. Certain vessels, especially in the right atrium, empty 

 into the cavity of the heart as small veins known as the vena minima 

 (or veins of Thebesius). Minute veins in the papillary muscles have 

 been described as opening into the ventricle at both ends. 



