sinus to ventricle be left, and the muscular connexions entirely severed, no wave passes. In contradistinction to cross-striated muscle, the structural unit of the heart is not also a functional unit, for the heart-cells are, from the earliest stage of development, joined together by branches into networks and bands so as to form one functional whole, and hence excitation of any one part leads to the contraction of the whole. The first part to begin to function ate in the embryo. is the venous end, and the waves of contraction passing thence spread over the developing ventricular segment. The muscle-cells of the ventricles are thicker, less sarcoplasmic and more clearly striated than the auricular muscle, which is more embryonic in structure. The contraction lasts longer in the ventricular than in the auricular muscle, while the automatic rhythm not only persists longer in the auricles, but is of greater frequency, as is clearly seen when the cavities of the heart are divided from each other. The venous orifices of the heart are least sensitive to injury, beat longest after death, and are the first to recover after arrest. Owing to the more powerful automatism of the venous extremity, the contraction normally proceeds thence, and. passing as a peristaltic wave over the auricles and ventricles, finally reached the arterial orifices. This peristaltic form of contraction is invariable in all periods of development and in all hearts, both of invertebrate and vertebrate animals. The peristalsis may, with difficulty, be artificially reversed by the application of a powerful rhythmic stimulus to the ventricular end. Antiperistalsis does not, however, take place easily, because the comparatively slow excitatory process in the ventricle has little effect on the auricular muscle. The latter, by initiating more rapid contraction-waves, over-dominates the former. The frequency of the whole heart is accelerated by warming the auricles, while the period of systole is alone shortened on warming the ventricles.
The sequence in the beat of the three chambers of the heart is attributed by Gaskell to the delay that occurs in the excitatory wave passing through the muscular connexions in the sino-auricular and auriculo-ventricular junctions. He showed that such delay could be imitated by moderately clamping a strip of heart muscle; the compressed part transmitted the wave less readily, so that the part above and below the clamp contracted in sequence.
Fig. 15.—The Right Auricle and Ventricle of a Calf's Heart, exposed to show the course and connexions of the auriculo-ventricular bundle. 1, central cartilage exposed by dissection; 2, the main bundle; 3, auricular fibres from which the main bundle arises; 4, right septal division; 5, moderator band; 6, a cusp of the tricuspid valve; 7, posterior group of the musculi papillaries; 8, orifice of the coronary sinus; 9, above orifice of the inferior vena cava (10); 11, orifice of the superior vena cava; 12, septal wall of the right auricle; 13, appendix of the right auricle; 14, septal wall of the infundibulum; 15, beginning of the pulmonary artery; 16, apex of the right ventricle. (After A. Keith, in Journal of Anatomy and Physiology.)
In the mammalian heart there has recently been discovered a remarkable remnant of primitive fibres persisting in the neighbourhood of the venous orifices (representing the sinus). These fibres are in close connexion with the vagus and sympathetic nerves, and form the sino-auricular node of A. Keith and Martin Flack. If this node is squeezed by a clamp, it prevents the effect of excitation of the vagus reaching the heart. The auricle and ventricles of the mammalian heart are connected through the septum by a remarkable bundle of muscle fibres which is believed to convey the excitatory wave from the one cavity to the other. The root of this auriculo-ventricular bundle lies in the right auricle, the main part is buried in the inter-ventricular septum; its branches and twigs are distributed to all parts of either ventricle; the papillary muscles and fleshy columns, in particular, receive a direct supply. The muscle fibres are of a peculiar type, known as the cells of Purkinje. By this bundle it is believed every part of the ventricle is brought into synchronous contraction. To its degeneration has been ascribed certain cases of disturbed cardiac rhythm, when the ventricle no longer follows the sequence of auricle. The evidence of such degeneration is, at present, not convincing.
The contraction of the heart, like that of other muscle, is accompanied by an electrical change. The part in contraction is at different potential to the part at rest. Thus an electrical wave accompanies the wave of contraction. This has been studied by means of the capillary, or the string,The electrical change of the heart. electrometer (Sir John Scott Burdon-Sanderson and Page, Einthoven, Gotch). The photographic records obtained with these instruments afford us a most beautiful method of recording the rhythm of normal and abnormal hearts in man, for they can be obtained by connecting the right hand and left foot of a patient with the instrument. Einthoven, by making use of the telephone wires, recorded in his laboratory the electrical changes of the hearts of patients seated in a hospital 2 m. away.
Fig. 16.—Electrical Changes of Heart. A, diphasic variation of auricle; R—V, diphasic variation of ventricle. R=base negative; V=apex negative to base. After auricular contraction the ventricular is delayed—an example of arhythmia. (Einthoven.) The string galvanometer is the best method for elucidating disorders of cardiac rhythm.
The heart during the period of systole is refractive to artificial excitation, but its susceptibility returns with diastole. The force and amplitude of any cardiac contraction depend on the previous activity of the heart and on such physical conditions as the degree of diastolic filling, the resistance to systolic outflow, temperature, &c., but are independent of the strength of the artificial stimulus so long as the latter is efficient. Owing to the refractory period, the slow rate of contraction and the independence of the amplitude of contraction on the strength of stimulus, the heart under ordinary conditions cannot be thrown, by rapidly repeated excitation, into a complete state of tetanic spasm. The refractory period can be shortened by heat (40° C.), or by calcium and sodium salts until tetanus is obtainable. The cardiac muscle is rich in sarcoplasm, and on this depends its power of slow, sustained contraction. The heart-muscle, besides rhythmically contracting, possesses “tone," and this tone varies with the conditions of metabolism, temperature, &c. Chloroform, for example, produces a soft dilated, strychnine, adrenalin or ammonia a tonically contracted heart. The mammalian heart ceases to beat at temperatures below 7° C. and above 44° C., and passes into “heat rigor” at 45° C.
Fig. 17.—The origins of pneumogastric and vasomotor systems are in medulla, that of the sympathetic in upper portion of cord. The arrows indicate direction of nerve currents. In the heart R represents a reflex centre, I an inhibitory centre and A an accelerating centre.
The Cardiac Nerves.—In 1845 the brothers Weber made the astonishing discovery that the vagus nerve, when excited, slowed or even arrested the action of the heart. This was the first roof of the existence of inhibitory nerves. The Meduna cardiac inhibitory nerves have since been found in all classes of vertebrates and in many invertebrates. Some years later v. Beyeld (1862) and Moses and Il’ya Cyon (1843–) discovered the existence of nerve fibres which, when excited, augmented and accelerated the beat of the heart. These nerves arise from 1–5 thoracic anterior spinal nerve roots and have their “cell, stations” in the first thoracic and inferior cervical ganglia, whence they pass to the heart partly in company with the cardiac branches of the vagus, and partly as separate twigs. The vagus cardiac fibres arise by the middle of the lowermost group of vagus roots, and have their “cell stations” in the ganglion cells of the heart. These ganglion cells lie chiefly in the sub-pericardial tissue in the posterior wall of the auricles between and around the orifices of the venae cavae and pulmonary veins and between the aorta and pulmonary artery. The minute structure of these ganglia and the terminations of the nerves have been studied particularly by Dogiel. The inhibitory fibres arise from a centre in the spinal bulb which is in tonic action and constantly bridles the heart's action. When the vagi are divided the frequency of the heart increases and the blood pressure rises. The vagus centre is reflexly excited by the inhalation of chloroform, ammonia or other vapour irritant to the air passages, also by the want of oxygen in the blood in asphyxia. It may be excited by irritation of the abdominal nerves, e.g. a blow on the abdomen, and by increased pressure in the cerebral vessels. The accelerator and augmenting fibres
