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 



m.d.U 



iiij.s.pc. 



iii.n.e. 



Fig. 6. Perspective view of pericardial floor, based on an isometric projection, to show the attachments of 

 the pericardial compressor, f.a.pc. floor of anterior pericardium; f.p.pc. floor of posterior pericardium; 

 h.v. hepatic valve; m.ao. aortic muscle; m.d.l.i, ist dorsal longitudinal muscle; tn.l.s.pc. lateral sub- 

 pericardial muscle; m.n.e. nauplius eye muscle; m.pc.c. pericardial compressor; m.pc.d. 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 



