THE CIRCULATION OF THE BLOOD. 191 



If there were only circular fibres forming the -ventricular wall, it is 

 evident that on systole the ventricle would elongate; if there were only 

 longitudinal fibres, the ventricle would shorten on systole; but there 

 are both. The tendency to alter in length is thus counterbalanced, 

 and the whole force of the contraction is expended in diminishing the 

 cavity of the ventricle; or, in other words, in expelling its contents. 



On the conclusion of the systole the ventricular walls tend to expand 

 by virtue of their elasticity, and a negative pressure is set up, which 

 tends to suck in the blood. This negative or suctional pressure on the 

 left side of the heart is of the highest importance in helping the pul- 

 monary circulation. It has been found to be equal to 23 mm. of mer- 

 cury, and is quite independent of the aspiration or suction power of the 

 thorax itself, which will be described in a later chapter. 



The musculi papillares prevent the auriculo-ventricular valves from 

 being everted into the auricle. For the chordae tendinese might allow 

 the valves to be pressed back into the auricle, were it not that when the 

 wall of the ventricle is brought by its contraction nearer the auriculo- 

 ventricular orifice, the musculi papillares more than compensate for this 

 by their own contraction holding the chords tight, and, by pulling 

 down the valves, adding slightly to the force with which the blood is 

 expelled. 



These statements apply equally to the auriculo-ventricular valves on 

 both sides of the heart; the closure of both is generally complete every 

 time the ventricles contract. But in some circumstances the tricuspid 

 valve does not completely close, and a certain quantity of blood is 

 forced back into the auricle. This has been called the safety-valve action. 

 The circumstances in which it usually happens are those in which the 

 vessels of the lung are already completely full when the right ventricle 

 contracts, as, e.g., in certain pulmonary diseases, in very active exertions, 

 and in great efforts. In these cases, the tricuspid valve does not com- 

 pletely close, and the regurgitation of the blood may be indicated by a 

 pulsation in the jugular veins synchronous with that in the carotid 

 arteries. 



(2.) TJie Semilunar. It has been shown that the commencement of 

 the ventricular systole precedes the opening of the semilunar valves by a 

 fraction of a second. This would seem to show that the intraventricular 

 pressure does not exceed the arterial pressure until the systole has actually 

 begun, for the opening of the valves takes place at once when there is a 

 distinct difference in favor of the intraventricular over the arterial press- 

 ure, and continues open only as long as this difference continues. When 

 the arterial begins to exceed the intraventricular pressure, there is, as it 

 were, a reflux of blood toward the heart, and the valves close. The dila- 

 tation of the arteries is, in a peculiar manner, adapted to bring this about. 



