Page:EB1911 - Volume 27.djvu/957

 circulation are thus far from simple. They resolve themselves mainly into a consideration of (1) the physiology of the heart; (2) the physical characters of the circulation; (3) the control of the heart and vessels by the nervous system. From Hill's Manual of Physiology, by permission of Edward Arnold. . 4.—Diaphragm of Chambers of Heart and Large Vessels. A, Vena cava, superior. B, Vena cava, inferior. C, Pulmonary artery. D, Aorta. E, Right auricle. F, Right ventricle. G, Left auricle, into which open the four pulmonary veins. H, Left ventricle.

A. Keith, in Journal of Anatomy and Physiology.

. 5.—Showing the Attachments of the Heart. a, a, auricular base of ventricle; c, c, aortic base of ventricles; d, d, arterial mesocardium; e, e, venous mesocardium; f, ascending aorta; g, pulmonary aorta: h, superior vena cava: i, inferior vena cava, perforating diaphragm and pericardium: l, m, n, structures at the root of the lung—bronchus, pulmonary artery, and pulmonary veins; o, vortex at apex; p, pectinate musculature of right auricle; r, superficial musculature of right ventricle.

The Action of the Heart.

The permanent position and general arrangements of the heart are described in a separate article, and it is only necessary here to allude to certain points of physiological importance The substance

of the heart is composed of a special kind of muscular tissue which must be regarded as a syncytium in which no distinct and separate cells occur. a complex plexus of branching and anastomosing fibres, forming one functional whole. The fibres are nucleated, have a cross-striated structure and are surrounded by delicate conneé tive tissue sheaths. The cross-striations are due to the primitive fibrils which as in skeletal muscle are differentiated into alternate doubly and singly refracting substances. These fibrils are embedded in a granular nucleated sarcoplasm. Between the bundles of fibres are thin layers of connective tissue containing closely spun networks of capillaries. The muscle of the auricles consists of a circular layer common to both and a deeper layer separate for each chamber. The auriculo-ventricular ring consists of connective tissue surrounding the auriculo-ventricular orifices and separating the auricular from the ventricular muscle with the exception of an important band, the auriculo-ventricular bundle. The superficial fibres of the ventricles appear to have origin in the auriculo-ventricular ring, to wind about the heart spirally and to end in the tendons of the papillary muscles or pass up to the ring again on the inner surface of the heart. The middle layers consist of bundles of fibres running more or less circularly round the ventricles. The greater part of the heart lies free in the pericardial sac. The pericardium is reflected from the wall of the sac on to the wall of the heart and attaches the heart at the point where the, venae cavae and aorta leave the sac. This part of the pericardium. gives a fixation point to the auricles, for it is attached to the roots of the lungs and thereby to the thoracic wall, to the diaphragm and to the structures at the root of the neck. On opening the chest the normal fulcra for the movements of the auricles are lost, and this renders it difficult to record the exact movements of the heart. The attached part of the heart is called the base, and the venous part of the base is the beginning and the arterial part the end of the tube, coiled on itself, from which in the embryo the heart develops. The longitudinal and circular muscle fibres of the ventricles are antagonists. The circular fibres by their contraction tend to lengthen the apex-base diameter, the longitudinal fibres resist this and the two together wring the blood out of the heart. The apex is maintained as a fixed point by, this antagonistic action, and thus the longitudinal fibres are enabled to expand the auricles by pulling down the floor of these chambers. This action is important, as it contributes to the filling of the auricles simultaneously with the emptying of the ventricles. Tracings of the jugular pulse give evidence of such action.

In the case of the auricles the longitudinal musculi pectinati not only help the circular, fibres to expel the blood, but draw up the base of the ventricle to meet its load of blood. Thus the base of the ventricular part (or floor of the auricles) is pulled up during auricular systole, and down during ventricular systole. The posterior and upper borders of the left auricle lie against the unyielding structures of the posterior mediastinum, the pulmonary arte and gronchi, the floor and anterior part in contact with the base of the ventricle and ascending aorta respectively. The latter parts alone are free to move during systole. Thus the left ventricular base is drawn up and the aorta back on auricular systole (A. Keith).

As regards the valves of the heart—(1) the tricuspid guards the right auriculo-ventricular opening, and consists of three flaps of fibrous tissue covered like all the internal surfaces of the heart with the smooth shining membrane, the endocardium. The flaps are continuous at their base, forming an annular of the membrane surrounding the opening. The bicuspid or mitral

consists of two cusps and guards the left auriculo-ventricular opening. The under surface and free edge of each cusp of these