Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/117

Rh VASCULAR SYSTEM 101 ontrac- ion. This shock is propagated to the apex of the ventricles and thus causes a vibration of the intercostal space. The elevation d is synchronous with the closure of the aortic valves and e with that of the pulmonary valves ; it is also apparent that these, valves are not closed at the same moment, but are separated in time by 05 to 09 of a second, an effect due, as already explained, to the greater blood pressure in the aorta than in the pulmonary artery. Finally, cf corresponds to the remaining part of the ventricular diastole, sature of Much discussion has taken place as to whether the contraction leart s of the heart is to be regarded as a simple contraction, like the &quot;twitch&quot; of a muscle obtained by a single stimulation, or as a tetanic contraction, like cramp, such as is caused by the application of a number of stimuli in rapid succession. It is true that many of the phenomena of a cardiac contraction resemble those of a skeletal muscle : thus, fatigue diminishes the amplitude and in creases the duration of the contraction ; and the effects of changes of temperature are similar. The period of latent stimulation of a cardiac muscle (one-third of a second) is much longer than that of skeletal muscle (one-hundredth of a second). The systolic contrac tion, as regards duration, is more like a tetanic spasm than a twitch, being from eight to ten times longer. The electrical phenomena, on the other hand, resemble those of a twitch more than those of tetanus. Thus, when the heart is examined with a sensitive galvano meter, and with the aid of the appliances described under PHYSIOLOGY (vol. xix. p. 24 sq.), there is a &quot;negative variation&quot; with each beat. The fact appears to be that, just as the heart muscle shows histo- logical characters intermediate between voluntary striated muscle and involuntary non-striated muscle, so it likewise shows physio logical properties partaking of both. The time occupied by cardiac movements has been measured by various observers by a study of tracings obtained from the impulse of the apex of the heart against the wall of the chest, as recorded by the cardiograph. If the velocity of the surface on which the tracing is obtained be known, and if a correct interpretation is given of the causes of the various parts of the curve, it is not difficult to determine approximately the time occupied by the phases of a cardiac revolution. Rollett gives the following results in fractions of a second as determined by Landois (Hermann, Handb. d. PhysioL, vol. iv. p. 157). Time of
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uove- neuts. Events in the Heart. Rate of Heart-Beat per Minute. 55 55 74-2 109-7 113-1 1. From beginning of pause to end of auri- Duration of Phases. 503 243 066 259 309 822 1-133 584 274 072 200 346 784 1-133 494 079 144 092 223 586 809 078 213 066 194 090 244 303 547 100 247 057 133 103 190 394 539 057 3. Relaxation of ventricle to closure of semilunar valves 4. From close of pulmonary valves to be ginning of pause 6. Between 2d sound and next 1st sound. . 7. Time from 1 to 4 inclusive (complete cardiac revolution) 8. From closure of aortic valves to closure of pulmonary valves Fre- In the adult man cardiac pulsations occur at the rate of 65 to 75 quency per minute. There is a certain relation between the amount of of cardiac blood in the circulation and the frequency of heart-beats. Thus, jmlsa- in the animal series, as the beats of the heart increase in fre- tions. quency the quantity of blood which passes per minute in one kilo gramme (2 2 tt&amp;gt;) of body weight also increases, as is shown in the following table by Yierordt : Quantity of Blood per Minute and per Kilogramme of Body Weight. Number of Pulsations per Minute. 55 Man 207 ,, 7 Dog 9 72 96 Rabbit 620 ,, 220 Guinea-pig 892 320 Quantity It has been determined, both by direct measurement and by cal- of blood culation based on the velocity of the blood in the aorta and the in. heart, transverse section of the orifice of that vessel, that from a heart of average size each left ventricular systole ejects about 180 grammes (6 - 35 07,.). This is the figure usually given, but it must be regarded merely as approximative. The amount may vary even in the same individual according to the state of vigour of the muscular walls of the organ. Sounds When the ear is applied over the cardiac region of the chest of a of heart, healthy man two sounds are heard, the one with greatest intensity over the apex and the other over the base of the heart. The dull long sound heard over the apex has received several names, such as the first, the long, the inferior, and the systolic sound, whilst that over the base, clearer, sharper, shorter, higher, has been called the second, the short, the superior, the diastolic sound. Suppose the heart sounds to be expressed by the syllables lupp dupp ; then the accent is on lupp (the first sound) when the stetho scope is over the apex thus, lupp dupp, lupp dupp and on dupp (the second sound) when over the base thus, lupp diipp, lupp dupp. There is a pause between the second sound and the next succeeding first sound, and a much shorter pause (almost in appreciable) between the first and second sounds : thus At apex lupp dupp (pause), lupp dup? (pause), lupp dupp. At base lupp dupp (pause), lupp dupp (pause), lupp dupp. These relations are well seen in fig. 11. Dr Walshe states that, if the cardiac cycle be divided into tenths, the first sound will last four-tenths, the short pause one-tenth, the second sound two -tenths, and the long pause thiee-tenths. There has been considerable dif ference of opinion as to the cause of the first sound. Some have supposed it to be due to vibrations of the auriculo- ventricular valves ; others believe that it is muscular, and due to the contraction of the ven tricles ; not a few have at tributed it to movements of the blood through the aortic and pulmonary ori fices ; whilst yet others have thought that it might FlG - H.- Scheme of a cardiac cycle after Gaird- i ,1 i, ,. f e nerandSharpey. The inner circle shows what be the result ot a fusion of evcnts occu % the heart) and the outer the these effects. It is cer- relation of the sounds and silences to these tainly not due to the shock events. of the heart against the chest wall, as it has been heard after re moval of the heart from the chest. The most likely view is that it is a muscular sound, varying in quality from the ordinary sound of a contracting muscle in accordance with the peculiar arrange ment of the cardiac fibres, and that this sound is modified by the vibrations of the tense auriculo- ventricular valves. The fact that the sound has been heard from an excised heart still pulsating but empty of blood strongly supports this view ; and there is further the pathological evidence that in cases where the muscular walls have been much weakened by fatty changes (as in the advanced stages of typhus, fatty degeneration, &c. ) the first sound may dis appear. No doubt exists as to the cause of the second sound : it is produced by the sudden sharp closure of the sigmoid valves. As already mentioned, the aortic and pulmonary valves do not close absolutely simultaneously. For practical purposes it is important to bear in mind what is happening in the heart whilst one listens to its sounds. With the first sound we have (1) contraction of the ventricles, (2) closure of the auriculo-ventricular valves, (3) rushing of the blood into the aortic and pulmonary artery, (4) impulse of the apex against the chest, and (5) filling of the auricles. With the second sound we have (1) closure of the semilunar valves from the elastic recoil of the aorta and pulmonary artery, (2) relaxation of the ventricular walls, (3) opening of the auriculo-veutricular valves so as to allow the passage of blood from auricle to ventricle, and (4) diminished pressure of apex against chest wall. With the long pause there are (1) gradual refilling of the ventricle from the auricle and (2) con traction of the auricle so as to entirely fill the ventricle. The sound of the tricuspid valve is loudest at the junction of the lower right costal cartilage with the sternum, of the mitral at the apex beat, of the semilunar valves at the aortic orifice in the direction of the aorta, where it is nearest to the surface, at the second right costal cartilage, and of the valves at the pulmonary orifice over the third left costal cartilage, to the left and external to the margin of the sternum. For an account of the mechanical work performed by the heart, see NUTRITION, vol. xvii. pp. 685-686. The heart is directly nourished by the blood flowing through its Nutri- cavities in some of the lower Vertebrates, as the frog ; but in the tion of hearts of larger animals, in which nutritional changes must be heart, actively carried on, there is a special arrangement of vessels or cardiac circulation. The coronary arteries originate at the aortic orifice in the region of the sinus of Valsalva, rather above the upper border of the semilunar valves, so that when the ventricle con tracts the mouths of these arteries are not covered by the segments of the valves. The branches of the coronary arteries, after dividing and again dividing, penetrate the muscular substance and end in a rich plexus of capillaries, which carry arterial blood to the struc ture of the heart. From these the radicles of the cardiac veins originate, and these veins cany the blood, now rendered venous, into the right auricle by the larger anterior cardiac veins and by numerous small veins constituting the foramina of Thebesius or the vense, minimse cordis. The coronary vein is dilated before enter ing the auricle, forming the coronary sinus, and at the junction of the vein with the dilated portion there is a valve consisting of