THE THORAX. 



177 



muscle and run between it and the external intercostal, fhe superior branch running 

 along the lower edge of the rib and the inferior branch running alcng 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 op- 

 posite the angle of the ribs it divides into superior and inferior branches which 

 unite with those from the internal mammary (arteria mammaria internet). 



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 wound- 

 ing 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 



c aorta 



Posterior intercostal arteries 



Anterior perfor- 

 ating arteries 



Anterior intercos- 

 tal arteries 



Internal mammary arteries 



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



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. 



MUSCLES. 



Covering the chest anteriorly are the pectoralis major and pectoralis minor 

 muscles. The serratus anterior (magnus] winds around its side and posteriorly, above 

 is the trapezius and below the latissimus dorsi. Beneath them are the erector spince 

 (sacrospinalis) muscles on each side of the spinous processes. 



The pectoralis major muscle arises from the sternal half of the clavicle, from 

 the sternum and costal cartilages as low as the sixth or seventh rib, and from the 

 i? 



