Such an outline (seen in the cut, page 22, with the angles rounded off) shows that the apex of the heart points downwards and towards the left, the base a little upwards and towards the right; that the greater part of it lies in the left half of the chest, and that the only part which lies to the right of the sternum is the right auricle. A needle introduced in the third, the fourth, or the fifth right intercostal space close to the sternum would penetrate the lung and the right auricle. A needle passed through the second intercostal space, close to the right side of the sternum, would, after passing through the lung, enter the pericardium and the most prominent part of the bulge of the aorta. A needle passed through the first intercostal space, close to the right side of the sternum, would pass through the lung and enter the superior vena cava above the pericardium. [Illustration: OUTLINE OF THE HEART, ITS VALVES, AND THE LUNGS.] 42. The best definition of that part of the præcordial region which is less resonant on percussion, was given by Dr. Latham years ago in his ‘Clinical Lectures.’ ‘Make a circle of two inches in diameter round a point midway between the nipple and the end of the sternum. This circle will define, sufficiently for all practical purposes, that part of the heart which lies immediately behind the wall of the chest, and is not covered by lung or pleura.’ =Apex of the heart.=--The apex of the heart pulsates between the fifth and sixth ribs, two inches below the nipple, and one inch to its sternal side. The place and extent, however, of the heart’s impulse, vary a little with the position of the body. Of this anyone may convince himself by leaning forwards, backwards, on this side and on that, feeling, at the same time, the heart. Inspiration and expiration also alter the position of the heart. In a deep inspiration it may descend half an inch, and can be felt beating at the pit of the stomach. 43. =Valves of the heart.=--The aortic valves lie behind the third intercostal space, close to the left side of the sternum. The pulmonary valves lie in front of the aortic behind the junction of the third costal cartilage, on the left side, with the sternum. The tricuspid valves lie behind the middle of the sternum, about the level of the fourth costal cartilage. The mitral valves (the deepest of all) lie behind the third intercostal space, about one inch to the left of the sternum. Thus these valves are so situated that the mouth of an ordinary sized stethoscope will cover a portion of them all, if placed over the sternal end of the third intercostal space, on the left side. All are covered by a thin layer of lung; therefore we hear their action better when the breathing is for a moment suspended. 44. =Outline of the lungs.=--Now let us trace on the chest the outline of the lungs, with as much precision as their expansion and contraction in breathing permit. (See the cut.)