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.
47. Reflection of pleura.—The reflection of the pleura from the wall of the chest on to the diaphragm corresponds with a sloping line drawn from the bottom of the sternum over the cartilages of the ribs down to the lower border of the last rib.
Since the pleura lines the inside of the last rib, a musket ball or other foreign body, loose in the pleural sac, and rolling on the diaphragm, might fall to the lowest part of the sac, which would be between the eleventh and twelfth ribs. The ball might be extracted here. The chest might also be tapped here, but not with a trochar, since a trochar would penetrate both layers of pleura, and go through the diaphragm into the abdomen.
The operation should be done cautiously, by an incision beginning about two inches from the spine, on the outer border of the ‘erector spinæ,’ on a level between the spines of the eleventh and twelfth dorsal vertebræ. The intercostal artery will not be injured if the opening be made below the middle of the space, which is very wide.[3]
THE BACK.
48. Median furrow.—In a muscular man, a furrow, caused by the prominence of the erector spinæ on each side, runs down the middle of the back. The lower end of the furrow corresponds with the interval between the spine of the last lumbar and that of the first sacral vertebra.
49. Spines of vertebræ.—A little friction with the fingers down the backbone will cause the spines of the vertebræ to be tipped with red, so that they can be easily counted, and any deviation from the straight line detected. Still it is worth remembering that the spine of the third dorsal is on a level with the commencement of the spine of the scapula—that the spine of the seventh dorsal is on a level with the inferior angle of the scapula—that the spine of the last dorsal is on a level with the head of the last rib.
Division of the trachea.—The division of the trachea is opposite the spine of the third, in some cases the fourth, dorsal vertebra. In front this division is on the level of the junction of the first with the second bone of the sternum.
The root of the spine of the scapula is marked by a slight dimple in the skin. This is on a level with the third intercostal space. A stethoscope placed on the inner side of this dimple would cover the bronchus, more especially the right, since it is nearer to the chest wall.
Make a man lean forwards, with his arms folded across the chest; this will make prominent the spines of the vertebræ. The lower border of the trapezius will guide you to the spine of the twelfth dorsal vertebra.