Part 6

e. When the arm is raised, the highest visible digitation of the serratus magnus corresponds with the sixth rib. The digitations below this correspond respectively with the seventh and eighth ribs.

f. The scapula lies on the ribs from the second to the seventh, inclusive.

g. The eleventh and twelfth ribs can be felt even in corpulent persons, outside the erector spinæ, sloping downwards.

h. One should remember the fact that the sternal end of each rib lies on a lower level than its corresponding vertebra. For instance, a line drawn horizontally backwards from the middle of the third costal cartilage at its junction with the sternum, to the spine, would touch the body, not of the third dorsal vertebra, but of the sixth. Again, the end of the sternum would be on about the level of the tenth dorsal vertebra. Much latitude must be allowed here for variation in the length of the sternum, especially in women.

39. =Interval below clavicle.=--Immediately below the clavicle we recognise the triangular interval between the pectoralis major and the deltoid. This space varies in different cases, depending on the distance between the muscles. It is important as a guide to the coracoid process and the axillary artery. In a case of injury to the shoulder, to ascertain whether the coracoid process is broken, carry the arm outwards, to put the deltoid and pectoral muscles on the stretch, and make manifest the space between their opposite borders. Pressing the thumb into the space we can feel the inner side of the coracoid process, the apex being under the fibres of the deltoid; thus it is easy to ascertain whether it be broken. Moreover, this space corresponds with the line of the axillary artery; here its pulsation can be distinctly felt, and here it can be compressed (but not easily, or for long) against the second rib.

40. =Internal mammary artery.=--The line of the internal mammary artery runs perpendicularly behind the cartilages of the ribs, about half an inch from the sternum. The perforating branch through the second intercostal space is generally the largest.

41. =Outline of heart on chest-wall.=--To have a general idea of the form and position of the heart, map its outline on the wall of the chest, as follows:--

a. To define the base draw a transverse line across the sternum corresponding with the upper borders of the third costal cartilages: continue the line half an inch to the right of the sternum and one inch to the left.

b. To find the apex, mark a point about two inches below the left nipple, and one inch to its sternal side. This point will be between the fifth and sixth ribs.

c. To find the lower border (which lies on the central tendon of the diaphragm), draw a line, slightly curved downwards, from the apex across the bottom of the sternum (not the ensiform cartilage) as far as its right edge.

d. To define the right border (formed by the right auricle), continue the last line upwards with an outward curve, so as to join the right end of the base.

e. To define the left border (formed by the left ventricle), draw a line curving to the left, but not including the nipple, from the left end of the base to the apex.

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.

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.)

45. The apex of each lung rises into the neck behind the sternal end of the clavicle and sterno-mastoid muscle as much as an inch and a half: in females rather higher than in males (30). From the sternal ends of the clavicles the lungs converge, so that their thin edges almost meet in the mesial line on a level with the second costal cartilage. Thus there is little or no lung behind the first bone of the sternum. From the level of the second costal cartilage to the level of the fourth, the margins of the lungs run parallel, or nearly so, close behind the middle of the sternum: consequently their thin edges overlap the great vessels and valves at the base of the heart.

Below the level of the fourth costal cartilage the margins of the lungs diverge, but not in an equal degree. The margin of the right corresponds with the direction of the cartilage of the sixth rib: the margin of the left, being notched for the heart, runs behind the cartilage of the fourth. A line drawn perpendicularly from the nipple would find the lung margin about the lowest part of the sixth rib. Laterally, i.e. in the axillary line, the lung margin comes down as low as the eighth rib: posteriorly, i.e. in the dorsal or scapular line, it descends as low as the tenth.

It should be remembered that, in a deep inspiration, the lung margins descend about one inch and a half.

In children the lungs are separated in front by the thymus gland. Allowance should be made for this. About the approach of puberty the thymus disappears.

46. =Anterior mediastinum.=--The direction of the anterior mediastinum is not straight down the middle of the sternum, but slants a little to the left, owing to the position of the heart. The right pleural sac generally encroaches a little upon the left, behind the middle of the sternum. A needle introduced through the middle of the sternum opposite the third or the fourth rib would go through the right pleura.