The intercostal spaces are occupied by the two intercostal muscles, with a fascia above them, one below, and one between them.

The external intercostal muscles run downward and forward. They begin at the tubercles of the ribs posteriorly and end at the costal cartilages anteriorly. They are continued forward to the sternum by the anterior intercostal membrane, formed by the fusing of the outer and middle intercostal fascias. The internal intercostal muscles go downward and backward. They begin at the sternum and end at the angles of the ribs. They are continued to the spine by the posterior intercostal membrane, formed by the fusing of the middle and internal intercostal fascias. The intercostal arteries come from both anteriorly and posteriorly.' The anterior intercostals come from the internal mammary for the upper five or six spaces and from the musculophrenic artery for the remainder. They arise either as a single trunk or as separate superior and inferior branches. At first they are between the pleura and internal intercostal muscle, but they soon perforate that muscle and run between it and the external intercostal, the superior branch running along the lower edge of the rib and the inferior branch running along the upper edge of the rib below. The aortic or posterior intercostal arises as a single trunk which passes between the external intercostal muscle and the pleura. Arriving opposite the angle of the ribs it divides into superior and inferior branches which unite with those from the internal mammary (arteria mammaria interna).

From the vertebrae out to the angle of the ribs the intercostal artery lies about midway between the ribs, hence it is liable to be wounded in paracentesis if the puncture is made too far back. It is for this reason that operations for draining the pleurae are performed anterior to the costal angles. The superior intercostal branches are larger than the inferior ones. They run under the lower edge of the rib above the space and are therefore protected from injury, particularly stab-wounds.

In opening the chest for empyema it is best to go about midway in the intercosal space and not too close to the lower edge of the rib on account of the liability of wounding the superior intercostal. The inferior branch is usually quite small and causes no serious hemorrhage. Intercostal bleeding may cause a haemothorax if the wound is small. It may be controlled, if the vessel is cut in performing the operation of paracentesis for empyema, by clamping with haemostatic forceps. If these are allowed to remain on a few minutes the bleeding often does not recur on their removal. If desired a ligature can be applied. If it is undesirable to rely on the clamp or ligature then the wound may be firmly packed with gauze or a piece of gauze may be depressed through the wound into the pleural cavity and then stuffed with more gauze, after which the tampon so formed is pulled firmly outward against the bleeding tissues.

Fig. 200.   Course and distribution of the intercostal arteries.

Fig. 200. - Course and distribution of the intercostal arteries.