Discovery reports (1929) Discovery reports discoveryreports21931inst Year: 1929 BLOOD SYSTEM 457 The first possibility is that the heart contracts as the pericardium enlarges, and vice versa. This appears impossible because of the nature of the cardiac valves. The aortic valve is simply a median split in the pericardial floor, the lips of which are muscular extensions of the pericardial dilator. Now the myofibrils of this muscle run upwards and spread out towards the attachment to the pericardial floor. Here a small band runs forwards (Figs. 2, 4 A) and splits into two as the myofibrils supp
Discovery reports (1929) Discovery reports discoveryreports21931inst Year: 1929 BLOOD SYSTEM 457 The first possibility is that the heart contracts as the pericardium enlarges, and vice versa. This appears impossible because of the nature of the cardiac valves. The aortic valve is simply a median split in the pericardial floor, the lips of which are muscular extensions of the pericardial dilator. Now the myofibrils of this muscle run upwards and spread out towards the attachment to the pericardial floor. Here a small band runs forwards (Figs. 2, 4 A) and splits into two as the myofibrils supporting the lips Fig. 6. Perspective view of pericardial floor, based on an isometric projection, to show the attachments of the pericardial compressor, floor of anterior pericardium; floor of posterior pericardium; hepatic valve; aortic muscle; , ist dorsal longitudinal muscle; lateral sub- pericardial muscle; nauplius eye muscle; pericardial compressor; pericardial dilator. of the aortic valve. The majority of the fibrils, however, converge and run upwards as the median sub-pericardial muscle. The direction of these fibrils can be established clearly with polarized light. From this arrangement it is clear that contraction of this muscle must pull the pericardial floor both downwards and backwards. This must tend to increase the pericardial space but, at the same time, the lips of the aortic valve will become tense and will be pulled close together in the middle line, thus shutting the valve. Similarly with the hepatic valves. Although the minute muscles of the latter appear to be isolated and not connected directly with any other muscle, it is reasonable
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