The lower intercostal arteries arise from the aorta on each side, and before they enter the space between the ribs give off a branch passing backward to the vertebral canal and the posterior muscles of the spine. The eleventh and twelfth intercostal arteries, covered at first by the pillar of the diaphragm, ascend on leaving the vertebræ to reach the under edges of the ribs, and are accompanied by a vein and nerve. The tenth intercostal artery, and those immediately above it, run almost horizontally, and nearly in the mid-spaces of the ribs, as far as their angles, at which part a small artery is commonly given off, which descends from the main trunk at an acute angle to the rib below, and may be injured in opening into the chest, and be perhaps mistaken, in operating, for the intercostal artery itself. From the angles each artery runs in a groove in the under edge of the rib as far as the anterior third, when they all become very much diminished in size, and, leaving the grooves, run in the middle of the intercostal spaces, until lost in their different anastomoses with the branches of the epigastric, phrenic, lumbar, and circumflexa ilii arteries.
In making an opening into the chest between the tenth and eleventh, or between the eleventh and twelfth ribs, the artery will not be injured, provided the opening be made below the middle of the intercostal space, which is wider between the eleventh and twelve ribs than between those above it. The vein is situated above the artery, and proceeds to the vena azygos major on the right, and to the smaller azygos vein on the left side.
The intercostal nerves are the anterior branches of the dorsal nerves, and lie below the arteries under the pleura upon the external intercostal muscles, until they approach the angles of the ribs, where they enter between the layers of the intercostal muscles.
It is worthy of observation that the pleura is necessarily continued over the inside of the twelfth rib to line the different attachments of the diaphragm, and that an incision may always be made into the chest above this point, if done carefully.
On removing the integuments of the back, covering the muscles and the lower ribs, the broad expanse of the latissimus dorsi muscle is brought into view, extending from the ilium and spine upward and outward, and covering all the parts of importance beneath in the operation to be described. On the removal of the lower part of this muscle the serratus posticus inferior is seen, of a somewhat quadrilateral form, arising by a thin aponeurosis common to it and to the latissimus dorsi, from the spinous processes of the three superior lumbar vertebræ and the two inferior dorsal, and proceeding upward and outward to be inserted by four flat, tendinous digitations into the four lower ribs.
If this muscle be separated from its origins and turned outward, or divided in the middle, and its two portions reflected, the posterior spinal or long muscles running in and filling up the groove or hollow of the side of the spine will now be distinctly seen, composed chiefly of the sacro-lumbalis and the longissimus dorsi muscles, sometimes called as a whole the erector spinæ or the sacro-spinal muscle. This, which forms a thick mass over the beginning of the tenth, eleventh, and twelfth ribs, is not to be divided or interfered with beyond a very few at most of its external fibers; the opening into the chest about to be made should begin at its external edge and go through the external intercostal muscle, which is now exposed on a plane below it.
The eleventh and twelfth ribs, unlike all those which precede them, except the first, have only one surface of articulation with the corresponding vertebræ, to which they are attached, instead of two facettes articulating—one with the body of the vertebra above, the other with that below. They form, particularly the twelfth, a more acute angle with the spine than the other, which gives to them their greater degree of obliquity, while the freedom of their cartilaginous extremities enables the twelfth, particularly, to be depressed or separated by a moderate force from the rib above to a greater extent than at any other part, by which means a foreign body of larger size may be removed from between them more readily than elsewhere.
349. Operation.—The eleventh and twelfth ribs having been distinctly traced, and the obliquity of their descent from the spine having been clearly made out, the patient ought, if possible, to be placed on a stool, with the upper part of the chest supported by a pillow on a table before him. An incision should then be made over the intercostal space between these ribs, three inches long and slightly curved, through the integuments down to the latissimus dorsi muscle, and as the mass of long spinal muscles is usually three inches in width, and can in general be seen, the incision should commence two inches from but between the spinous processes of the eleventh and twelfth vertebræ, and be continued obliquely or diagonally downward in the course of the interspace between these ribs. The latissimus dorsi and the serratus posticus inferior muscles having been divided at the upper part where they cover the longissimus dorsi or the long spinal muscular mass alluded to, its edge becomes apparent; from this point the latissimus and the serratus are to be further divided downward. The external intercostal muscle being thus exposed, its fibers should be scratched through or separated in the middle of the interspace between the ribs, which can now be seen as well as felt. A director should be introduced below the muscle, on which it may be carefully cut through, as well as any fibers of the internal intercostal muscle which may extend as far as the wound thus made. The pleura will then be exposed, and if the cavity of the chest contain fluid in any quantity, it can scarcely fail to project in such a manner as to convey to the finger the assurance of its being beneath. An opening may then be carefully made into it at the upper part of the incision close to the external vertical fibers of the spinal mass of muscles, at the moment of inspiration, and on the existence of fluid being ascertained by its discharge, the opening should be enlarged by a director previously introduced under the pleura, the patient being desired to draw a full breath at the time, in order that the diaphragm may descend as low as possible. If there should not be any fluid in the chest, the diaphragm, in ascending during expiration, may be applied to the inside of the pleura lining the chest as high even as the fifth rib, counting from above, and might easily be divided with the pleura, if great care were not taken to make the opening during the process of inspiration.
In all cases of wounds of the chest, in which auscultation points out the presence of a ball rolling loose on the diaphragm, this operation should be performed for its removal, and may save the life of the sufferer. It would, perhaps, have done so in the case of Sir Robert Crawford. At a later period the presence of a foreign body, perhaps, can only be known by the sounds or defect of sounds which may be observed at the back part of the chest, in which the ball or other foreign bodies lodge or become enveloped by matters confining them in that situation.