the right ventricle forms the greater portion of the anterior surface of the heart, and the left of the posterior. Size and dimensions of the heart.-The capacity of each of the four cavities of which the heart is composed is approximately the same, and may be put at 100 cc., or about 3 ounces. The walls of the different chambers vary greatly in thickness. The FIG. I. SECTION THROUGH THE BASE OF THE HEART, SHOWING THE P=pulmonary orifice; A=aortic orifice; M = mitral opening; T=tricuspid opening 25 average thickness of the auricular walls is about 1 mm., or of an inch. The wall of the left auricle has a maximum thickness of 3 mm., or of an inch, and the wall of the right auricle 2.5 mm., or of an inch. The thickness of the wall of the left ventricle inch), and of the right 10 ranges between 4 mm. and I cm. (to ventricle between 2 mm. and 4 mm. (12 to inch). Relative position of the cardiac orifices and valves. The relative position of the various orifices with their valves is most readily appreciated by means of a transverse section taken just above the ventricles, so that the structures exposed can be viewed from above. The accompanying diagram illustrates such a section (Fig. 1). Pulmonic orifice. It will be observed that the conus arteriosus crosses the aortic opening obliquely from right to left, whence it happens that the pulmonic orifice is carried in front of and slightly to the left of the aortic. Aortic orifice. -The orifice of the aorta lies in front of and between the two auriculo-ventricular openings. The aortic and pulmonic orifices are each guarded by a valve consisting of three semilunar flaps, which are differently arranged at the two openings. At the aortic orifice one flap is anterior and to the right, one anterior and to the left, and the other posterior and to the right; while at the pulmonic opening one is anterior and to the left, one posterior and to the left, and the third posterior and to the right. Mitral opening. The left auriculo-ventricular or mitral orifice is situated immediately behind and to the left of the aortic opening. It is guarded by a valve composed of a small posterior and a larger anterior curtain, the latter of which separates the mitral from the aortic orifice. The close proximity of the large anterior flap of the mitral valve to the orifice of the aorta is of some clinical importance, and will be again referred to. Tricuspid opening. The right auriculo-ventricular or tricuspid opening is closed by a valve consisting of three segments, of which one is placed anterior and to the left, a second to the right, and the third posteriorly. It is worthy of notice that the dimensions of the aortic and pulmonic orifices are anatomically incapable of any alteration at the time of the closure of the semilunar valves. On the other hand, the size of the auriculo-ventricular openings is very much diminished during systole, in order to enable the valvular curtains to come into accurate apposition. This difference in the mechanism of closure of the two sets of openings has, as will be seen, important pathological and clinical bearings. The following measurements (taken from Macalister's Text Book of Human Anatomy) are of some interest as showing the relative size of the orifices of the heart :: Topography of the heart.-It has already been mentioned that nearly two-thirds of the bulk of the heart lie to the left of the median line of the body. The upward extent of the organ is represented on the surface of the chest by a horizontal line drawn slightly below the level of the second costal cartilages. The heart lies to the left of a vertical line drawn downwards from the sternal end FIG. 2. RELATION OF THE HEART AND AORTA TO THE RIBS AND STERNUM of the right clavicle, and to the right of an oblique line considerably convex upwards and outwards, drawn from the second costal cartilage about an inch from the left sternal border to the upper border of the sixth costal cartilage, 9 cm. or 3 inches from the mid-sternal line. It lies above a line, very slightly concave upwards, drawn from the upper border of the sixth rib, 9 cm. from the mid-sternal line, to the lower border of the cartilage of the right fifth rib, close to the sternal edge. The width of the heart at the level of the fourth costal cartilage is about 4 inches, of which 1 inches lie to the right of the midsternal line, and 3 inches (nearly) to the left. Area of deep cardiac dulness.-The whole of this area marks out what is known as the region of "deep cardiac dulness," and corresponds approximately with the absolute size of the heart so far as its anterior aspect is concerned. Area of superficial cardiac dulness.-By the term superficial cardiac dulness is meant the extent of the cardiac surface uncovered by the margins of the lungs, and in contact, through the pericardium, with the chest wall. This area is included in a triangle, which is bounded by the mid-sternal line, by a line drawn from a point on the mid-sternal line, opposite the level of the upper border of the fourth costal cartilages, to the lower border of the fifth left costal cartilage, at its junction with the rib, and by a line drawn hori. zontally inwards from this point to the mid-sternal line. For practical purposes the area of superficial cardiac dulness may also be defined by a circle 2 inches in diameter, drawn from a point midway between the left nipple and the end of the gladiolus (Latham). The relation of the different chambers and orifices of the heart to the area of deep cardiac dulness will now be described. Right auricle.-The right auricle lies behind the sternal ends (about 1 inches from the mid-sternal line) of the third, fourth, and fifth costal cartilages, the second, third, and fourth intercostal spaces, and the right sternal edge. The apex of the right auricular appendix is situated in the middle line opposite the level of the upper border of the third costal cartilages. Left auricle.—The only part of the left auricle that is visible from the front is its appendix, which is placed behind the second left interspace and upper part of the third costal cartilage about 11⁄2 inches from the left sternal edge. Right ventricle.-The greater portion of the anterior surface of the heart consists of the wall of the right ventricle, which, measured along the left edge of the sternum, extends from just above the third rib to the lower border of the sixth costal cartilage. Its upper part is formed by the conus arteriosus, and is uncovered by lung. Left ventricle. The left ventricle forms the left border of the heart as high as the upper border of the third rib, and also the apex of the organ. The apex of the heart in adults is situated in the fifth intercostal space, 9 cm. or 3 inches from the mid-sternal line. A diagrammatic representation of these relations is shown in the preceding figure. Tricuspid opening. The right auriculo-ventricular or tricuspid opening is behind the middle of the sternum opposite the fourth costal cartilages. Mitral opening.-The left auriculo-ventricular or mitral orifice lies behind the sternum, to the left of the middle line, on a level with the upper border of the fourth costal cartilages. Aortic orifice. The aortic opening is situated behind the sternum to the left of the middle line on a level with the lower border of the third costal cartilages. The ascending portion of the aorta extends from this point to the level of the upper border of the second right costal cartilage at its junction with the sternum. Pulmonic orifice. The pulmonic orifice lies behind the upper border of the third left costal cartilage at its junction with the FIG. 3. THE POSITION OF THE VALVULAR ORIFICES, AND THEIR RELATION P=pulmonary orifice; A=aortic orifice; M = mitral orifice; T=tricuspid orifice P (within circle) = pulmonary area; A (within circle) = aortic area sternum. The vessel ascends vertically from this point to the level of the second left costal cartilage, behind which it bifurcates. The auriculo-ventricular sulcus.--The auriculo-ventricular sulcus is represented by a line drawn from the upper border of the |