circulation'are thus far from simple. They resolve themselves mainly into a consideration of (1) the physiology of the heart; 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 nucleatedisarcoplasm. 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

FIG. 3.-The Thoracic Viscera. In this diagram the lungs are turned to the side, and the pericardium removed to display the heart. a, upper, u/, lower lobe of left lung; b, upper, b', mlddle, b”, lower lobe of right lung; c, trachea; d, arch of aorta; e, superior vena cava; f, pulmonary artery; g, left, and h, right auricle; k, right, and l, left ventricle; m, inferior vena cava; n, descending aorta; I, in nominate artery; 2, right, and 4, left common carotid artery; 3, right, and 5, left subclavian artery; 6, 6, right and left in nominate vein; 7 and 9, left and right internal jugular veins; 8 and IO, left and right subclavian veins; II, 12, 13, left pulmonary artery, bronchus and vein; 14, 15, 16, right pulmonary bronchus, artery and vein; 17 and 18, left and right coronary arteries. (2) the physical characters of the circulation; (3) the control of the heart and vessels by the nervous system. . .

circularly. round the ventricles. The eater part of the heart lies free in the pericardia sac. The pericar ium is reliected from the wall of the sac on to the wall of the heartwfand attaches the heartvat the-point where the, venae cavae and aorta" leave the sac. This part of the peri-Cilrdium. gives a fixation point to the auricles, or it is attached to. the roots of the 1. lungs and thereby to .the t oracic wall, to the diaphragm and” to ' the structures” at the root of the neck. Orrgopenin the chest the "" = normal fulcra for the moifements'¢o£';";the auricles are lost, and this renders it difficult to record the exact movements of the heart. Theattached part of the heart is called the bdse, 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

F IG. 6.-Cavities of the Right Side of the Heart. a, superior, and b, inferior vena cava; c, arch of aorta; d, pulmonary artery; e, right, and f, left auricular appendage; g, fossa ovalis; h, Eustachian valve;, mouth of coronar vein; l, m, n, cusps of the tricuspid, valve; o, 0, papillary muscles; p, semi lunar valve; g, corpus Arantii; r, lunula. From Hill's Manual of Physiolo y, by permission of Edward Arnold: F 1G. 4.~Diagr-am of Chambers of Heart and Large Left auricle, into which open the four pulmonary veins. Left ventricle.

A. Keith, in Journal of Anatomy and Physiology.

FIG. 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: Z, m, n, structures at the root of the- lung bronchus, pulmonary artery, and pulmonary veins; 0, vortex at apex; 12, pectmate musculature of right auricle; r, superficial muse

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 posteriormediastinum, the pulmonary arte and gronchi, the floor and anterior part in contact with the base ollythe 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 culature of right ventricle. The Action of the Heart.

The permanent position and general arrangements of the heart

are described in a separate artic e, and it is only necessary here to allude to certain points of physiological importance; The substance i, tissue covered like all the internal surfaces of the heart with the smobth shining membrane, the endocardium. The fiaps are continuous at their base, forming an annular of the membrane surrounding the opening. The bicuspid or mitral beam consists of two cusps and guards the left auriculo-ventricular opening. The under surface and free edge of 'each cusp of these