Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/122

Rh 106 VASCULAR SYSTEM tent than the arteries. Numerous anastomoses exist between veins and even between superficial and deep veins, so that if the flow of blood be obstructed in one direction it readily finds a passage in another. The circulation in the veins depends (1) on inequality of blood-pressure, the pressure being much less in the veins than in the arteries ; (2) on muscular action compressing the veins, and thus, in consequence of the valves found in many veins opening towards the heart, forcing on the blood in the direction of that organ ; (3) on the movements of respiration, inspiration, as already seen, favouring the flow of blood in the great veins towards the heart ; and (4) on the suction-like action of the right auricle, and in the case of the lungs that of the left auricle, drawing the blood towards the heart. During venesection, muscular action increases the flow of blood from the divided vein ; hence the use of the barber s pole, which was grasped by the patient during bleeding by the barber-surgeon of old. The flow of blood in veins is continu ous, or nearly so ; when, therefore, a vein is cut, it does not &quot;spurt&quot; as an artery does, but it &quot;wells out&quot; in a stream. Occa- There is normally no pulse in veins ; but sometimes a pulse may sioual be observed in the veins of the neck, isochronous with the auricular pulse in systole, when there is an obstruction to the passage of blood from veins. the right auricle into the right ventricle. Pulse-tracings (fig. 21) taken in these circumstances are very similar to those of the cardiac impulse. In this tracing the part ab represents the right auricular contraction. During the systole of the right ventricle the tricuspid valve closes, and, if it be insufficient, that is, if it does not close properly, a positive wave is transmitted along the superior vena cava to the jugular (be in the pulse-tracing). The closure of the pulmonary is indicated at e. During the diastole of the right auricle and ventricle the blood flows to the heart and the curve descends, /. It has also been pointed out by Friedreich that a pulse in the jugular vein does not necessarily mean insufficiency of the tri cuspid valve but a weakened condition of the valve in the jugular vein itself, as the pulse will not be propagated into the jugular, even in cases of insuffi ciency of the tricuspid valve, if the jugular valve be perfect. If there is great obstruction at the mitral orifice, a venous pulse may also be observed, which is associated with engorgement of the light auricle. Sometimes a pulse in the veins occurs when there is d pulse from in sufficiency of the tricuspid valve. (Fried reich.) Venous sounds. such rigidity from atheroma in the walls of the great Fin. 21. Trac- vessels as to destroy the elastic influence of these in s of venous parts, and at the same time such a degree of dilata tion of the arterioles and capillaries as to admit of the onward propulsion of the movement caused by the heart s contraction. Lastly, a pulse may occur when the blood-pressure rises and falls suddenly, as in insufficiency of the aortic valves, and when the arterioles are much dilated. Towards the close of life, when the heart is feeble and effusion may be taking place into the pericardium, a venous pulse may be observed. If a stethoscope be placed at the root of the neck above the collar bones, and on the right side in particular, a whistling, rushing, or blowing sound will be heard. This is the bruit de (liable, familiar to physicians. If heard without pressure being made by the stetho scope it is abnormal, as it occurs in conditions of anamiia from almost any cause ; but it may be heard in a healthy person when pressure is applied and when the head is turned to the opposite side. It is held to be due to the vibration of the blood in rushing from the contracted portion of the common jugular vein into the more dilated part of this vessel. During the. auricular diastole and during inspiration it is more marked, as the blood then flows more rapidly in the veins towards the heart. Phenomena of General Circulation. Prime Having described the structure and functions of the organs con- factors cerned in the circulation, namely, heart, arteries, capillaries, and influen- veins, we are in a position to consider the phenomena of the circula- cing cir- tion as a whole. Consider the organs of the circulation as a closed culation. -system of tubes, over-filled with blood ; when the tubes are in a state of rest, it is evident that, if the blood be uniformly diffused and under the same pressure, it will remain motionless and in equilibrium. When the pressure is changed at any point, as occurs when the left ventricle contracts and throws blood into the arterial system, the blood will move from the part where the pressure is higher to where it is lower ; in other words, there will be circulation as a consequence of the difference of pressure. When the heart stops beating, the blood continues to flow more and more slowly until the difference of pressure is equalized, and then there is no circula tion. Each stroke of the heart throws as much blood into the arteries as flows into the heart from the veins ; the orifices of the veins at the heart are more distensible than the beginnings of the arteries, and consequently the arterial pressure rises more rapidly than the venous pressure diminishes, and thus the beating of the heart raises the mean pressure throughout the arterial system. The circulation is therefore influenced by two factors, (1) the heart, as regards number, strength, and volume of beats ; and (~2) the amount of resistance in the arterioles. Modifications of these influences are the pressure and the velocity of the blood. As the blood is circulating through the vessels under the influ- Blood- ence of the action of the heart, it exerts a certain pressure or ten- pressui sion, the existence of which is shown by the jet of blood which spurts in out on the puncture of an artery, and the amount of which is indi- vessels cated by the height to which the jet is propelled. An instrument for measuring this pressure, termed a kymograph, has been devised by Ludwig. But this apparatus, owing to the inertia of the mass of mercury, the medium used, can only register mean blood- pressure, and the more delicate variations escape notice. Fick in 1864 attempted to register these smaller fluctuations by means of a curved spring-kymograph, consisting of a hollow spring, which is made to oscillate by variations of pressure communicated to the interior. .Small portions of a tracing taken with the mercurial manometric kymograph of Ludwig are shown in fig. 22. From this 70 / L/v i i -A A Q 05 y j fj T n ^ vj * v V r r W /

E V x f E l k 2 V N N rf Fro. 22. Mercurial kymographic tracing from carotid of dog, showing form of curve on a large scale. The figures on the left represent mm. of mercury. (Marey.) it will be observed that there is (1) an increase and diminution of blood-pressure with each cardiac beat, as is shown in the smaller curves, and (2) an increase and diminution produced by respiratory movements, the increase occurring chiefly during inspiration and the decrease chiefly during expiration, as is indicated by the larger waves. It is evident also that all the smaller curves have the same general character, and that they reveal nothing as to varia tion in pressure during individual beats. To show such variations, Fick s kymograph must be used, when a tracing will be obtained in which slight oscillations of pressure in the down-stroke of each separate beat may be observed. By employing the different forms of kymograph the following conclusions have been arrived at. (a) Arterial Pressure. (1) The pressure diminishes from the heart Arterial to the capillaries. (2) It attains its maximum in the ventricle at pressun the moment of systole, and its minimum in the auricle at the moment of diastole, at which time also the pressure in the auricles and in the great veins may be negative, that is, below atmospheric pressure. (3) The mean blood-pressure in the large arteries of large mammals and of man is equal to that of a mercurial column 5 5 to 6 3 inches in height. (4) The blood-pressure in various animals has been ascertained to be as follows, the results being expressed in terms of a column of mercury of the indicated number of inches in height. Carotid of horse, 6 34 ; carotid of dog, 5 89 (Poiseuille). Carotid of horse, 4 8 to 8 42 ; carotid of dog, 5 89 ; carotid of goat, 4 64 to 5 31 ; carotid of rabbit, 3 54 ; carotid of fowl, 3 46 to 673 ; aorta of frog, 86 to T14 ; gill artery of pike, 1 38 to 3 3 (Volkmann). Carotid of dog, 5 12 to 7 48 (Ludwig). Carotid of calf, 6 97 ; carotid of sheep, 6 65 ; carotid of goose, 6 38 ; carotid of stork, 6 34 ; brachial artery of pigeon, 6 57 ; carotid of cat, 5 9 ; brachial artery of man during operation, 4 33 to 472 (Faivre). Anterior tibial artery of boy, 3 93 to 6 3 (E. Albert). It no doubt varies even in animals of the same species. (5) The arterial pressure at any given point undergoes periodic variations, increasing at the instant of ventricular systole and diminishing during diastole ; they are, most marked in the arteries near the heart. (6) These periodic variations may be observed in the intermittent spurting of an artery when it is punctured. (7) It is necessary to distinguish between the mean arterial pressure at any point of an artery and the mean pres sure of the blood in the whole arterial system, which can only be obtained by taking the mean of the pressures in many different arteries at various distances from the heart. (8) The mean arterial pressure depends directly on the quantity of blood in the arterial system, and consequently on the total calibre of the system, so that any diminution of calibre, produced mechanically or by nervous influences, will increase the mean arterial pressure. (9) The mean arterial pressure increases with the energy of the beats of the heart. (10) The blood-pressure becomes greater with increased and acceler ated action of the heart, sometimes with plethora, and after an increase in the amount of blood, such as occurs after a full meal or after transfusion of blood, whilst it becomes smaller during dimin ished or enfeebled action of the heart, in anaemia, and after haemor rhage or excretions from the blood by the skin, kidneys, or bowels (Landois). (11) The pressure is affected by the degree of contrac tion or of dilatation of the blood-vessels, according as they are influenced by the nervous system. (12) The pressure is increased in cases of sclerosis or hardening of the arterial walls, in lead poisoning, after injection of ergotin (which contracts the small artcrioles) or of digitalis (which acts on the heart), where there is granular or contracted kidney, and in cardiac hypertrophy with dilatation. And (13) the pressure is diminished in fever, in chlorotic antenna, in phthisis, and by severe haemorrhage.