THE ARRANGEMENT OF THE MUSCULATURE OF THE HEART 267 



of fibers. Both are directed more transversely then the superficial 

 layers. The former encircle the left cavity and the latter the right 

 cavity, and finally surround the large blood-vessels at the base of the 

 heart. 



These two longitudinal layers, form, so to speak, a sling-like 

 support for the circular fibers which are especially numerous on the 

 left side, and give an unusual volume and strength to this compart- 

 ment. It must be evident that the circular coat is the most important 

 dynamic factor, because its constrictor action serves to lessen the 

 lumen of the ventricular cavity in a most decided manner, thus giving 

 rise to the pressure which is required to drive the blood through the 

 system. It should be emphasized, however, that although each ven- 

 tricle is constructed in such a way that it forms a muscular unit, the 

 joint action of the two is assured by certain, strands of fibers which pass 

 from side to side and envelop both compartments. 



On contraction, each ventricular mass of tissue assumes a rounded 

 outline so that the two compartments become sharply differentiated 

 from one another by a groove which extends obliquely downward 

 from a point above and on the right side to a point below and on the 

 left. Moreover, in accordance with the general direction of the fibers 

 of the outer coat, the entire ventricular mass is turned at this tune 

 slightly around its longitudinal axis so that the apical center is rotated 

 from left to right and forward. For this reason, a more extensive 

 area of the left side of the heart is brought into view during this period ; 

 and naturally, only the left ventricle then presents itself below the 

 interventricular groove, because the apex is formed solely by the mus- 

 culature belonging to this compartment. 



The Arrangement of the Valves. With the exception of the ap- 

 pendix auriculae, the cavity of the auricle presents a perfectly smooth 

 internal surface. In the ventricles, on the other hand, open spaces 

 are encountered solely below the orifices of the aorta and pulmonary 

 artery. The former is designated as a rule as the aortic vestibule 

 and the latter as the conus arteriosus. The remaining space of each 

 ventricle is rendered rugose and uneven by numerous projecting bundles 

 of muscle-tissue which appear in the shape of (a) columns raised hi 

 relief from the wall, (6) as isolated cords of tissue stretching directly 

 through the cavity, and (c) as free conical and nipple-shaped elevations 

 projecting for a short distance into the lumen of the cavity. The first 

 are known as columnar carnece. Their function seems to coincide with 

 that of the general mass of the cardiac tissue. The second, called 

 moderator bands, are found most frequently in the right cavity. They 

 arise as a rule from the interventricular septum and are inserted in 

 the outer wall. Obviously, their purpose is to prevent an excessive 

 outward movement of the latter and an undue distention of the cavity 

 as a whole. The third, commonly referred to as the papillary muscles, 

 are in functional relation with the principal mass of the cardiac mus- 

 culature and serve as points of attachment for the chordce tendinece, 



