cartilage of the sixth rib on the right side, at its junction with the sternum, to the upper edge of the third left costal cartilage close to the sternal border.

The interventricular sulcus.--The anterior interventricular groove corresponds with an oblique line, slightly concave inwards, drawn from a point half an inch inside the apex, to the upper border of the third left costal cartilage, 1 inches from the left sternal edge.

It must be borne in mind that the foregoing landmarks are subject to considerable variation in different individuals, according to age, muscular and bony development, etc. In children the apex of the heart is situated a space higher than in adults, i.e. in the fourth intercostal space, and further to the left, so that it is often found in the nipple line, or even slightly outside this limit.

Aorta.-The first or ascending portion of the aortic arch extends from the lower border of the third left costal cartilage to the upper border of the second right costal cartilage at its junction with the sternum. The transverse portion of the arch runs across the sternum below the level of a line drawn horizontally through the middle of the manubrium, to the upper border of the fourth dorsal vertebra.

The third or descending portion of the arch lies to the left of the body of the fourth dorsal vertebra.

CHAPTER II

PHYSIOLOGY

Cardiac Cycle-Time relations and sequence of the various events which comprise Cardiac Cycle-Cardiac Impulse-Its position and cause-Sounds of the Heart-Their cause and duration-Nervous Supply of the Heart

THE term cardiac cycle is applied to that sequence of events which is included in each successive beat of the heart. It is composed of a contraction or systole of the auricles, a contraction or systole of the ventricles, and a pause or period during which all four chambers of the heart undergo relaxation, and then remain in a state of passivity (i.e. a condition in which the muscular walls neither contract nor relax) until the commencement of the succeeding cardiac cycle. This pause is called diastole.

The phases of contraction and relaxation on the right and left sides of the heart are, under normal conditions, exactly synchronous in the corresponding chambers.

The normal rate of the heart's action may be taken to be seventyfive beats per minute, so that each complete cardiac cycle is about eight-tenths() of a second in duration. The times occupied by the individual phases are approximately as follows:

It must be clearly understood that the clinical uses of the terms "systole" and "diastole" apply solely to these conditions in the ventricles. Consequently "systole," as applied to the phases of the cardiac cycle, includes the ventricular contraction, and part of

-6 Secs

the auricular relaxation; while "diastole" comprises the ventricular relaxation, part of the auricular relaxation; the passive interval, and the whole of the auricular contraction (see diagram Fig. 4).

FIG. 4.

•4 Secs.

DIAGRAMMATIC REPRESENTATION OF THE CARDIAC CYCLE

By the successive contractions of the auricles and ventricles the blood passes from the former into the latter, and from the latter into the arterial trunks. A reflux is prevented by the interposition of valves between the auricles and ventricles, and at the orifices of the aorta and pulmonary artery. The flaps of these valves come together and close the openings which they guard so soon as the blood pressure in front of them becomes greater than in the chamber immediately behind them. There is no valvular mechanism to prevent a reflux of blood from the auricles into the large veins during the auricular contraction, but immediately before and during the auricular systole there is a similar contraction of the muscular walls of those portions of the large veins which lie nearest the heart, and a consequent narrowing of their lumen, so that the blood takes the path of least resistance, and passes through the auriculoventricular openings into the ventricles.

2Secs.

Blood passes into the ventricles during the whole of the ventricular diastole largely in consequence of the positive pressure which prevails in the veins and auricles, but in some measure, no doubt, owing to the negative pressure which obtains in the ventricular chambers, by reason of the active dilatation of their walls. The final process of filling the ventricles is completed by the auricular systole.

The time relations and sequence of the various events which comprise the cardiac cycle are illustrated by the preceding diagram. The outer and inner circles represent the movements of the auricles and ventricles respectively. The radial and circular lines between the circles represent the contraction and relaxation of the auricles and ventricles respectively. The passive interval is left blank, while each eighth part of the circumference of the circles represents one-tenth of a second. The most constant feature of the cardiac cycle, with different rates of heart beat, is the duration of the ventricular systole. Hence it follows that an increase in the rate of the heart's action is obtained chiefly by means of the shortening of the diastole.

The cardiac impulse.-The cardiac impulse is the visible and palpable pulsation of the heart against the chest wall. It is synchronous with the systole of the ventricles, and can be most distinctly seen and felt in the fifth left intercostal space, about half to one inch on the sternal side of the vertical nipple line. The site. and force of the cardiac impulse vary somewhat with the position of the body.

The cause of the cardiac impulse is briefly as follows. In the state of rest, during diastole, the heart lies with its axis directed obliquely downwards, so that the apex of the organ (in the upright position at all events) is in contact with the chest wall at the point mentioned above. During systole the heart, besides being tilted slightly upwards, moves forwards and to the right, whereby the apex is brought into closer contact with the thoracic parietes at a time when the wall of the ventricles suddenly becomes tense and hard. Furthermore, the antero-posterior diameter of the heart is increased during systole, and in this way the anterior surface of the ventricles is also brought nearer to the chest wall. The total effect of these changes is to bring the apex and adjacent portion of the cardiac wall, during systole, into somewhat violent contact with the thoracic parietes in the position of the apex beat.

The cardiac impulse, therefore, is the local displacement of the thoracic parietes produced by the more or less forcible impact of the apex and adjacent portion of the hardened and rigid cardiac wall during the ventricular systole.

A lengthening of the long axis of the heart is not concerned in the production of the apex beat, inasmuch as this diameter of the organ remains apparently unaltered during systole and diastole.

If the heart's impulse be allowed to impinge on the end of a lever, or on the membrane of a tambour in one of the many forms

cf cardiograph, a tracing or impulse curve may be obtained, which will be considered more fully later (see p. 94).

The following is a normal cardiogram :

The sounds of the heart. On listening over the region of the heart two sounds may be distinguished, which have been likened to the pronunciation of the syllables lubb-dup. The first sound, as indicated by the consonous syllable lubb, is comparatively dull, deliberate, and prolonged; while the second sound, represented by dup, is sudden, sharp, and short. The interval between the first and second sounds is very short, while that between the second and succeeding first sound is relatively of considerable duration. By direct observation it has been proved that the first sound is synchronous with the ventricular systole, and the second sound with the closure of the semilunar valves. The first sound of the heart is caused partly by the tension vibration of the segments of the auriculo-ventricular valves, consequent on the sudden closure of their orifices; partly by the so-called muscular sound, which is produced by the vibration of the muscular wall of the ventricles, consequent on their contraction; and partly by the vibration of the mass of blood in the ventricles and of the chorda tendineæ.

The second sound is due partly to the tension vibration of the semilunar cusps, consequent on the simultaneous and sudden closure of the aortic and pulmonic valves, and partly to the vibration of the column of blood in the aorta and pulmonary artery.

The duration of the cardiac sounds, and more especially of the pauses between them, is subject to considerable variation, but the average times of these incidents, and their relation to the events comprising the cardiac cycle, are represented in the accompanying diagram. (Fig. 6.)

The nerves of the heart.-The nervous supply of the heart is derived from two sources, namely, from the pneumogastric or vagi, and sympathetic nerves.