516 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



During the diastole of the auricles the blood streams into them from the 

 large venous trunks which are in connection with the base of the heart, 

 the propelling force being the pressure of the blood in the veins and the 

 aspiration exerted by the lungs. Soon the elastic tension of the walls 

 of the dilated auricles becomes sufficiently great to balance the forces 

 which cause the entrance of blood into the auricles; but before the 

 entrance of blood into the auricles is entirely arrested, the ventricles, 

 which had, up to this point, been in systole, and thus prevented entrance 

 of blood from the auricles, now relax, the auriculo-ventricular valves 

 are forced open by the pressure of blood in the auricles, and the ven- 

 tricles dilate, not only through the aspiration of the lungs, but in virtue 

 of the elasticity of their own walls. 



The auricles now pass into systole, and by the pressure of the con- 

 traction of their muscular walls force the blood from the auricles through 

 the auriculo-ventricular openings into the relaxed and dilating ventricles. 

 The blood passes from the auricles into the ventricles, and not back into 

 the veins, because this is the direction in which the moving blood-cur- 

 rent meets with the least resistance. We have seen that by the opening 

 of the auriculo-ventricular valves the bottom falls out of the auricles and 

 the dilatation of the empty ventricles exerts a negative pressure on the 

 contents of the auricles. At the same time the contraction of the mus- 

 cular fibres of the auricles serves somewhat to constrict the openings of 

 the veins, and the pressure of the blood in the venae cavae, supported by 

 the valves in the inferior vena cava, offer a sufficient resistance to prevent 

 regurgitation into the veins. 



The blood continues to flow from the auricles into the ventricles 

 until the propelling force of the contracting auricles is balanced by the 

 elastic tension of the dilated ventricles or by commencing ventricular 

 systole, exit of blood from the ventricles being prevented by the closed 

 semi-lunar valves at the openings of the aorta and pulmonary artery. The 

 A r entricles now being filled, systole commences, the closure of the auriculo- 

 ventricular valves prevents regurgitation into the auricles, and, the force 

 of the ventricular contraction being greater than the pressure of the blood 

 in the aorta and pulmonary artery, the semi-lunar valves are forced open, 

 and the ventricles empty themselves completely into these vessels. The 

 ventricles then relax, regurgitation from the great arteries being pre- 

 vented by the closure of the semi-lunar valves, the ventricles fill them- 

 selves from the auricles, and the process goes on as before. 



3. Thk Hydraulic Principles op the Circulation. — The physical 

 principles concerned in the movements of the blood through the arteries 

 of the animal body are largely governed by the purely physical laws of 

 hydraulics. Before, therefore, we attempt to explain the movements 

 of the blood, a glance at the most important of the physical principles 01 



