BLOOD SYSTEM 459 



During systole these muscles relax. An antagonistic muscle common to both of 

 them is the first dorsal longitudinal muscle. Contraction of this muscle will pull back 

 the hinder wall of the pericardium which has been pulled forwards during diastole. 



The relaxation of the pericardial dilator relieves the tension on the valves, and so 

 allows blood to be pumped by the heart into the aorta and in smaller quantities into 

 the gut parenchyma. 



Blood must at this stage pass into the pericardium to replace that forced out by the 

 heart. Some will pass in direct from the marginal sinuses, but not at any great rate, 

 as such blood must be sucked through the network of minute channels in the valves. 

 I believe that it passes in chiefly through the posterior pericardium. In the floor of 

 this part of the pericardium lie the lateral sub-pericardial muscles. These curve upwards 

 from their position of origin in the ectoderm over the floor of the posterior pericardium 

 and down again to attach to and merge into the floor of the anterior pericardium 

 (Figs. 2, 4 A, 5, 6). Hence, on contraction, each must pull down its middle portion, 

 that is, the portion lying over the floor of the posterior pericardium, and so convert 

 this space into a tube leading directly into the anterior pericardium. 



It is possible that, to a slight extent, as the floor is depressed, the roof of the posterior 

 pericardium is lifted up by the contracting first dorsal longitudinal muscle. From 

 Figs. 2 and 4 A it can be seen that the commencement of this muscle slopes 

 markedly upwards, and so must tend to lift up the body wall covering the posterior 

 pericardium. 



To summarize the suggested interaction of heart and pericardium, during diastole 

 both pericardial dilator and compressors contract. During systole these muscles relax 

 and the lateral sub-pericardial muscles contract, thus allowing blood to enter the peri- 

 cardium from the main body cavity. 



The further passage of blood from the heart through the vessels into the body cavity 

 can, I think, be predicted with a certain degree of accuracy. 



The roof of the aorta is attached to the pericardial floor. Since, during systole, the 

 latter is raised the roof of the aorta must also be lifted up and, in this way, the volume 

 of the anterior part of the aorta is enlarged and accommodates the blood forced out of 

 the heart. This follows necessarily if the suggested working of the heart and pericardium 

 is accepted. As will be seen later, I believe that, during this phase, the hinder part of 

 the aorta is occluded by its roof being pulled down on to its floor by contraction of all 

 the aortic muscles (Fig. 7 A). 



At the commencement of diastole then, the anterior aorta will be swollen with blood. 

 The pericardial dilator will now commence to contract and the aortic musculature will 

 relax. This will cause three things. The aortic valve will close. The pericardial floor 

 will be pulled downwards on to the anterior part of the aorta and the aortic tendon and 

 hence the roof of the hinder part of the aorta will be raised. The aorta will thus tend 

 to be occluded anteriorly and opened up posteriorly. Hence blood will be forced back- 

 wards into the hind end of the aorta, and the anterior part of the supraneural ring 

 (Fig. 7 B, C). 



