48 CIRCULATION" OF THE BLOOD. 



the tricuspid valve. The lungs are still farther protected by the sufficiency of the mitral 

 valve, which prevents regurgitation from the left ventricle. In the systemic circulation, 

 extravasation of blood would not be followed by any serious results, and the circulating 

 fluid is made to pass through a considerable extent of elastic vessels, before it is distrib- 

 uted in the tissues. The value of this reasoning of course depends upon the simple ques- 

 tion whether or not there be any conditions of the circulation under which regurgitation 

 at the right auriculo-ventricular orifice can occur, the tricuspid valve being normal. 

 Judging from the amount of pressure required to produce regurgitation at this orifice in 

 our experiments upon bullocks' hearts, it does not .seem probable that a " safety-valve 

 function" actually exists; for the force required is much greater than could be exerted 

 by the right ventricle under any circumstances. 



Action of the Aortic and Pulmonic Valves. The action of the semilunar valves is 

 nearly the same upon both sides. In the intervals of the ventricular contractions, they 

 are closed and prevent regurgitation of blood into the ventricles. The systole, however, 

 overcomes the resistance of these valves and forces the contents of the ventricles into the 

 arteries. During thit, time, the valves are applied, or nearly applied, to the walls of the 

 vessel ; but so soon as the ventricles cease their contraction, the constant pressure of the 

 blood, which, as we shall see hereafter, is very great, instantaneously closes the openings. 



The action of the semilunar valves can be exhibited by cutting away a portion of the 

 ventricles in the heart of a large animal, securing the nozzles of a double syringe in the 

 aorta and pulmonary artery, and forcing water into the vessels. In performing this ex- 

 periment in 1864, we noticed that, while the aortic semilunar valves oppose the passage of 

 the liquid so effectually that the aorta maybe ruptured before the valves will give way, a 

 certain degree of insufficiency exists, under a high pressure, at the orifice of the pulmo- 

 nary artery. It is not probable, however, that the pressure of blood in the pulmonary 

 artery is ever sufficient to produce regurgitation when the valves are normal. 



It is probable that the corpuscles of Arantius, which are situated in the middle of each 

 valvular curtain, assist in the accurate closure of the orifice. The sinuses of Valsalva, 

 situated in the artery behind the valves, are regarded as facilitating the closure of the 

 valves by allowing the blood to pass easily behind them. 



Sounds of the Heart. If the ear be applied to the praecordial region, it will be found 

 that the action of the heart is accompanied by certain sounds. A careful study of these 

 sounds and of their modifications in disease has enabled the practical physician to distin- 

 guish, to a certain extent, the conditions of the heart by auscultation. This increases 

 the interest which attaches to the audible manifestations of the action of the great central 

 organ of the circulation. 



The appreciable phenomena which attend the heart's action are connected with the 

 systole of the ventricles. It is this which produces the impulse against the walls of the 

 thorax, and, as we shall see farther on, the dilatation of the arterial system, called the 

 pulse. It is natural, therefore, in studying these phenomena, to take the systole as a 

 point of departure, instead of the action of the auricles, which we cannot appreciate 

 without vivisections; and the sounds, which are two in number, have been called first 

 and second, with reference to the systole. 



The first sound is absolutely synchronous with the apex-beat. The second sound 

 follows the first with scarcely an appreciable interval. Between the second and the 

 first sound, there is an interval of silence. 



Some writers have attempted to represent the sounds of the heart and their relations 

 to each other, by certain syllables, as, " lubb-dup or lull tub ; " but it seems unnecessary 

 to attempt to make such a comparison, which can only be appreciated by one who is 

 practically acquainted with the heart-sounds, when the sounds themselves can be so 

 easily studied. 



