Furthermore, it belongs to every part of the heart, but there is a descending scale of this automatic power, from the sinus where it is highest to the lower parts of the ventricle where it is very slight (Gaskell). The normal sequence of contraction of the four parts is determined by the natural rhythm of these parts, but in the whole heart it is impossible for the ventricle to contract at its own rhythm because before it is ready to beat again, after a preceding contraction, it receives an impulse from the auricle. In the same way, the auricle never beats at its own rhythm ; it is always subordinated to the faster rate of impulses coming from the sinus. If, however, the ventricle is electrically stimulated at a rate slightly faster than the beat of the sinus, the normal sequence of contractions becomes reversed, the ventricle now contracting first and the sinus last. All recent experiments tend to confirm the myogenic theory of the heart beat.
(2) The Mammalian Heart.—Although the mammalian heart has no sinus venosus, its contractions are as regular, and as inde pendent of the nervous system as those of the lower vertebrates. Both in the heart in situ and in the excised heart the two auricles contract together, and after a short interval there follows the con traction of the two ventricles.
In the mammalian heart, within the region where the superior vena cava opens into the right auricle, there lies a club-shaped formation known as the sino-auricular or the S-A node; func tionally it is identical with the sinus of the amphibian and reptilian heart. The node is composed of slender fusiform cells with little striation. It can be considered as definitely proven that, in the normal heart, the sinus serves as the centre in which the stimulus for the cardiac contraction originates. The S-A node is, however, by no means the only place in which the impulses for the heart can originate. The seat of these rhythmically recurring impulses may shift to some other portion of the heart since, as in the frog's heart, the function of rhythmicity is potentially present in every part of the heart, and the S-A node governs the rate of the whole heart only in virtue of its faster rate of discharge of these impulses. On this account the S-A node has often been described as the "pace-maker" of the heart. Other centres in which the impulses sometimes originate, and which in certain cases may gain mastery over the whole heart, are known as ectopic centres It has been shown that a very distinctive system of muscle fibres lies enclosed within its own sheath beneath the endocardium. This system is known as the conductive of the heart. It begins as a few strands of fibres in the region of the coronary sinus; these strands converge in a thickening which is known as the auriculo-ventricular or the A-V node; it is composed of the same type of cells as the S-A node. The two nodes are not con
nected with each other, but are divided by the ordinary contractile elements of the auricle. From the A-V node, a thin bundle of tissue passes through the auric ulo-ventricular septum towards the ventricle. This bundle was first described by His, and is known as the A-V bundle. On penetrating the intra-ventricular septum, the bundle divides into two branches, which pass to the right and left ventricle respec tively. The two branches in turn divide and subdivide, forming an extensive arborization on the inner surface of the ventricles (fig. 4).
Various parts of the conductive system may become centres of ectopic rhythms. If the S-A node is destroyed or injured, or if the irritability of the A-V node is increased, the latter assumes the role of initiating the heart beat. Many nervous and other influ ences may modify the relative irritability of the two nodes, and a shift of the pace-maker from one to the other may occur tem porarily even under normal conditions. Destruction of the His bundle is equivalent to a complete functional separation of the ventricles from the auricles. The ventricles develop a rhythm of their own, the idio-ventricular rhythm. Both ventricles however continue to contract together, the centre for their activity being localized in the higher part of the bundle. It is probable that still lower centres may assume a rhythmicity, but it is doubtful whether these can ever dominate the rhythm of the whole heart. Their significance consists rather in the fact that they serve to disturb some other dominant rhythm.
Thus we see that every part of the heart may serve as a centre of origin of an impulse. The natural rate of the discharges of these centres is in the following descending order : the S-A node, the A-V node, the bundle, its branches and muscle. If several centres are active simultaneously, the rate of the heart as a whole will be dominated by that centre which discharges most frequently.
The "Normal" Heart Rate.—The normal heart rate presents considerable variations in different individuals and in different species of animals. For man it may be estimated at 68 to 76 beats per minute, and for woman 74 to 8o; but the normal rate for some individuals may be much lower (5o) or much higher (9o). Small animals as a rule have a higher heart rate than large animals, e.g., elephant 25-28, horse and ox 36-50, sheep 6o-8o, dog 100-120, rabbit 150-18o, mouse zoo; small birds like the canary have the extremely high rate of i,000 beats per minute. Usually in man, under normal conditions, the heart rate declines with age. While at birth it is about 140, it is Ioo–I o at the age of 5; in childhood it is about 9o, and in the adult about 7o. In old age it accelerates slightly and becomes about 80.