33. As a rule, the right half of the chest is slightly larger than the left. Of ninety-two persons of the male sex and good constitutions, seventy-one had the right side the larger; eleven the left; ten had both sides equal. The maximum of difference in favour of the right was one inch and a quarter. The measurements were made on a plane with the nipple. 34. =Peculiarities in the female.=--The chest of the female differs from that of the male in the following points:--Its general capacity is less; the sternum is shorter; the upper opening is larger in proportion to the lower; the upper ribs are more movable, and therefore permit a greater enlargement of the chest at its upper part, in adaptation to the requirements of pregnancy. 35. The top of the sternum is on a level with the second dorsal vertebra; and the available space between the top of the sternum and the spine is hardly more than two inches.[2] 36. =Parts behind first bone of sternum.=--There is little or no lung behind the first bone of the sternum, the space being occupied by the trachea and large vessels as follows:-- The left vena innominata crosses the sternum just below the upper border. Next come the great primary branches of the arch of the aorta. Deeper still is the trachea dividing into its two bronchi opposite the junction of the first and second bones of the sternum. Deepest of all is the œsophagus. About one inch from the upper border of the sternum is the highest part of the arch of the aorta, which lies on the bifurcation of the trachea. 37. The course of the arteria innominata corresponds with a line drawn from the middle of the junction of the first with the second bone of the sternum, to the right sterno-clavicular joint. When the artery rises higher than usual into the neck, its pulsation can be felt in the fossa above the sternum. 38. =Rules for counting the ribs.=--In fat persons it is often difficult to count the ribs; hence the following rules may be useful:-- _a._ The finger passed down from the top of the sternum soon comes to a transverse projection, slight, but always to be felt, at the junction of the first with the second bone of the sternum. This corresponds with the level of the cartilage of the second rib. _b._ The nipple of the male is placed, in the great majority of cases, between the fourth and the fifth ribs, about three-quarters of an inch external to their cartilages. _c._ The lower external border of the pectoralis major corresponds with the direction of the fifth rib. _d._ A line drawn horizontally from the nipple round the chest cuts the sixth intercostal space midway between the sternum and the spine. This is a useful rule in tapping the chest. _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.