Page:EB1911 - Volume 27.djvu/961

Rh heard, caused by the blood rushing through the vessel narrowed by the pressure of the instrument. The fluid escapes into a wider portion of the vessel beyond the point of pressure, and the sound is caused by the eddies set up there throwing the membranous wall of the vessel into vibration. Such a sound is heard over an aneurism. The pacental bruit heard during pregnancy is a sound of this kind, arising from pressure on the uterine arteries. In cases of insufficient aortic valves a double blowing murmur may be heard, the first being due to the rush of blood, into the vessel, and the second to the regurgitation of the blood back into the ventricle. These murmurs are produced by eddies of blood setting the membranous parts into vibration. . 14.—Scheme of a Cardiac Cycle. The inner circle shows what events occur in the heart, and the outer the relation of the sounds and silences to these events.

Occasionally a murmur is produced by the displacement of air in the bronchial vessels by the beat of the heart, and may simulate the murmur of aortic incompetence. By placing a stethoscope over the jugular vein on the right above the collar bone a murmur is heard, the bruit de diable, particularly if the subject turn his head to the left. This is held to be due to the vibration of the blood in the jugular vein rushing from the dilated to the contracted part. It is more marked during auricular diastole and during inspiration.

In the lower vertebrates, as the frog, the heart is directly nourished by the blood which fills the cavities in its sponge-like structure. In the warm-blooded vertebrates there is a special arrangement of coronary vessels. The two coronary arteries (right and left) originate at the root of the aorta from the sinuses of Valsalva. Their branches penetrate the muscular substance and end in a rich plexus of capillaries. From these arise the radicles of the coronary veins which open into the right auricle by the coronary sinus and other small veins. These openings are valved. The heart in contracting exerts a greater pressure than that of the coronary arteries, and so arrests the flow in these during the height of systole, and squeezes the blood within the coronary capillaries and veins on into the right auricle. On diastole the coronary system fills again. Sudden occlusion of any large part of the coronary arteries produces irregular and inco-ordinate contractions, followed by death of the heart. Gradual occlusion of the coronary arteries by degenerative changes in advanced life is one of the causes of the distressing form of cardiac distress known as angina pectoris. The work of the left ventricle is calculated by the formula W=VP+mv2, where V=volume of blood in c.c. expelled per beat, P =mean pressure in aorta, m =mass of the blood expelled on systole, and v=the velocity imparted to it. The volume of the output has been determined directly by inserting the stromuhr in the ascending aorta (Robert Adolf Tigerstedt), and indirectly by determining (1) how much oxygen is absorbed per minute, (2) the difference in the oxygen content of the arterial and venous blood, (3) the number of heart beats. If 1000 c.c. of oxygen are absorbed from the air breathed in a minute, and the arterial blood contains 10% more oxygen than the venous, it is clear that 100 × 100 c.c. of blood must have passed through the lungs in that time, and if the heart beat 100 times, the output for each beat would be 100 c.c. From the determinations made on animals the output is calculated for man to be 60–100 c.c. The velocity of the output can be calculated if the volume of the output is known, the duration of the period of output, and the diameter of the aorta. The pressure is measured with a manometer. The velocity is much greater at the orifice than in the aorta, for the blood can flow from the aorta during the whole cardiac cycle, while the whole of it must esca through the orifice into the aorta during the period of output. The worlé spent on maintaining the velocity is not, however, more than of the whole and is generally neglected in the calculation. The output is not greater than 60–100 c.c. (3 oz.) (Tigerstedt, Nathan Zuntz), and the mean arterial pressure in a healthy man, determined by the sphygmometer, is not more than 110 mm. of mercury (L. Hill). The work of the right heart can be reckoned to be that of the left, for the pressure in the pulmonary artery does not exceed 30 mm. The total work of the heart during the day may be taken as equal to 20,000 kilogr.-metres, and this would be equivalent to 50 calories out of the total 2500 calories which a man takes in as food. A labourer does about 150,000 kilogrm.-metres of external work a day. The work of the heart is increased two or three times over during severe muscular labour. It has been estimated that the heart requires per diem, to maintain its energy, an amount of solid food (water-free) equal to the weight of solids in the heart itself, i.e. about 60 grms. of sugar or proteid. 30 c.c. of blood must be circulated per minute through the coronary arteries of a dog to maintain the vigour of the heart.

The use of oxygen per grm. of weight per minute is high for the heart. Thus for the whole body of the dog there was used ·017 c.c. per grm. per min., for the heart .045–·083, and for the active secretory glands ·07–1·0 (Barcroft and Dixon). It has long been known that the heart of frog or tortoise can be kept beating normally for hours after removal from the body, if it is provided with an artificial circulation of blood or a suitable solution of, salts. Sydney Ringer worked out the necessary ingredients of this solution to be The excised mammalian heart can be kept beating in the same. way provided the nutritive fluid is oxygenated and the heart kept at body temperature. A solution containing one-third defibrinated blood and two-thirds Ringer's salt solution is most suitable. A mammalian heart thus was restored to activity 7 days after death. The beat of the heart of a child was restored 20 hours after death from pneumonia. The excised heart of a cat was kept, beating for 4 days. The heart of a monkey was restored after freezing the body of the animal. The nerves of the excised heart retain their action for some time if the nutritive fluid is immediately circulated through the coronary arteries. Thus the heart's action can be conveniently studied when taken from the body of a mammal.

The cause of the heart beat has naturally been one of the most continued objects of inquiry, and the point of view shifts with each advance of our experimental methods, and the wider extension of the inquiry throughout the animal world. H. Allen in 1757 was the first to announce that the activity the heart is not dependent on its connexion with the

nervous system. The excised heart, properly fed, continues to beat. The heart of a dog continued to work effectively and the animal to keep in health for months after division of all the nerves passing to the heart. The heart, it is true, is controlled and influenced constantly by the nervous system-attuned to the general needs of the body-but this control is not essential to life. The above do, when exercised, became fatigued quickly, owing to the lack of the nervous control of the heart. When in 1848 Robert Remak discovered that groups of nerve cells are contained in the heart of the frog, the causation of the beat was attributed to the activity of these ganglia.

Confirmation of this view was found in the experiment of Hermann Stannius which demonstrates that the apex of the heart ceases to beat rhythmically if physiologically separated from the rest of the heart by ligature or momentary application of a clamp. The sinus, on the other hand, which contains ganglion cells, continues its beat as before when separated. Further experiment has shown that the beat of the heart cannot be ascribed to the rhythmic activity of the ganglion cells, which in the mammalian heart ie scattered in the base of the heart, in the neighbourhood of the venous opening and in the auriculo-ventricular groove. That this is so is shown by the fact that every strip of heart muscle, whether free of ganglion cell or not, is capable of rhythmic activity under suitable conditions (Walter Gaskell, 1847–, Theodor Wilhelm Engelmann, Alfred Wm. Porter). The inherent power of rhythmic contraction' is most clearly seen in the embryonic heart, for the pulsation of the chick's heart became visible by the 24th to 48th hour of incubation, while the migration of the ganglion cells into the heart from the sympathetic system does not take place until the sixth day (His.). The heart muscle is pervaded by a network of nerve fibrils, and the supporters of the neurogenic theory have had to fall back upon this network as the cause of the beat. The “myogenic" theorists place the causation in the muscle itself.

The pulsating “umbrella” of the jelly-fish is formed of a network of nerve fibril and contractile elements, and this can be excited to contract by irritating any one of the sensory endings of the nervous network which are situated on the edge of the “umbrella.” In the manifestation of a “refractory period” the “umbrella” behaves like the heart. Against this view we may cite the experiment of Julius Bernstein (1839), who clamped off the apex off the frog’s heart to destroy the physiological continuity, kept the animal alive till the nerve network had degenerated and then found the apex could be mechanically excited to contract. Moreover, skeletal muscle-fibres can be thrown into rhythmic contraction by the application of a suitable solution of salts (Wilhelm Biedermann, 1854), and it is probable that heart muscle is excited to rhythmic activity by such means. At any rate the beat is profoundly affected by varying slightly the nature and percentage of salts supplied in the nutritive fluid. Carlson has recorded experiments upon the heart of the horseshoe crab (Limulus) which show that its beat at any rate depends on the integrity of the median nerve (and its ganglion cells) which runs down the heart. On the other hand, Gaskeil has shown that any small bridge of heart muscle left connecting the auricle and ventricle of the tortoise heart will transmit the wave of contraction, while if the nerve passing from