48 CIRCULATION OF THE BLOOD. 



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

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

 circulation, the capillaries are less delicate ; 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 distributed in the tissues. It is evident that, on 

 the left side, there is no necessity for such a provision, and it does not exist. 



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 this 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, it will be noticed that, while the aortic semilunar valves oppose the passage of 

 the liquid so effectually that the aorta may be ruptured before the valves will give way, 

 a considerable degree of insufficiency exists, under a high pressure, at the orifice of the 

 pulmonary artery. There is at this orifice a safety-valve function as important as that 

 ascribed by King to the tricuspid valve. It is evident that the slight insufficiency at the 

 pulmonic orifice may be even more directly important in protecting the lungs than the 

 insufficiency of the tricuspid valve. The difference in the sufficiency of the semilunar 

 valves on the two sides is fully as marked as between the auriculo-ventricular valves ; 

 and it is surprising that, since the observations of King, this fact, which we demonstrated 

 for the first time in 1864, has not attracted the attention of physiologists. 



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 prsecordial 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, " lull-dup or lubb-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. 



