CIRCULATION OF THE BLOOD. 33 



the ventricle in such manner that their edges are in contact, cusp 

 to cusp. As the ventricle contracts more violently, pressure is 

 brought to bear upon the valve, so that not only is the edge in 

 contact, but also portions of the surfaces of the cusps. These 

 valves are of considerable area, and are guyed in position by the 

 chordae tendinede, which spring from the papillary muscles, so that 

 eversion of the valve into the auricle is impossible. (2) The 

 semilunar valves form a guard against the return of blood to the 

 ventricle at the pulmonary and aortic openings of the ventricles. 

 These valves are forced open by the ventricular contraction, and 

 through them the blood rushes to distend the elastic walls of the 

 large arteries. The pressure of the blood under this elastic grasp 

 is sufficient to throw the cusps of the valves into action. The 

 corpora Arantii are useful in making a perfect closure of the valve, 

 though not absolutely essential. A part of the weight of this 

 pressure is borne by the thick ventricular wall, from the outer 

 edge of which the artery springs, while the valves are attached to 

 the inner edge. 



What is meant by the safety-valve action of the tricuspid valve ? 



Under some circumstances the tricuspid valve does not entirely 

 close, but allows a certain amount of regurgitation of blood. This 

 is in conditions of disease or of violent exertion, in which the lung 

 capillaries are overcharged with blood. This leakage of the valve 

 is conservative, by relieving the pressure upon the delicate capilla- 

 ries of the alveolae. Pulsation in the jugular veins indicates this 

 regurgitation. The condition is not pathological, and with altered 

 conditions disappears. 



What terms are used to describe the alternate contraction and 

 relaxation of the heart ? 



Systole, a contraction ; diastole, a relaxation. 



What is meant by the period of repose ? 



Between the contractions of the heart-muscle there is a percep- 

 tible pause, which has been called the period of repose. If the 

 time of a cycle be divided into five parts, the systole of auricles 

 will employ 1 part ; systole of ventricles, 2 parts ; period of repose, 

 2 parts. The accuracy of this division is not absolute, for, whatever 

 the pulse-rate, the ventricular systole consumes nearly four-tenths 

 of a second. 

 3 Phy. 



