94 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. 42. The anterior thoracic wall seen from within. The pleura has been removed. 



FIG. 43. The right intercostal region. In the upper of the three intercostal spaces represented the pleura is still 

 intact; in the second it has been removed; in the third, the internal intercostal muscle as well as the pleura has been 

 taken away. 



arteries) and the comes nervi phrenici artery, which runs upon the pericardium in company with 

 the phrenic nerves. The artery arises as the first branch of the subclavian, crosses the sternoclav- 

 icular joint, and runs downward on the inner surface of the thoracic wall, one centimeter from 

 the margin of the sternum, to the sixth intercostal space, where it divides into its terminal branches, 

 the musculo phrenic and the superior epigastric arteries (the latter anastomose with the inferior 

 (deep) epigastric from the external iliac). The most important branches are the anterior inter- 

 costal, which anastomose with the intercostal arteries from the aorta, and the perforating, which 

 pass anteriorly between the costal cartilages and are of considerable size in the female during 

 lactation. In the exposure of the internal mammary artery from without in the second, third, 

 or fourth intercostal spaces, the following structures are divided: The skin, the superficial 

 fascia, the deep fascia, the pectoralis major muscle, the intercostal fascia, and the internal in- 

 tercostal muscle. From within, the artery is covered only by the pleura; to the inner side 

 of the artery there is usually a single vein. Lower down the venae comites lie to either side of 

 the artery, and the vessels are separated from the pleura by the triangularis sterni muscle (Fig. 

 42). Injuries of the artery in the upper intercostal spaces are consequently more apt to lead to 

 effusions of blood into the pleural cavity (hemothorax) than similar injuries lower down. The 

 deep position of the artery makes it necessary to always bear in mind the possibility of hemo- 

 thorax after injuries in this region, even when the external hemorrhage is slight. Although the 

 vessel is so well protected that it is not frequently injured, many a person has bled to death 

 into the pleural cavity because the hemorrhage was not recognized. Upon the left side an in- 

 jury of the artery in the fourth or fifth intercostal space may lead to hemorrhage into the peri- 

 cardial cavity, since in this situation the vessel lies upon the pericardium instead of upon the 

 pleura. 



At the upper margin of the sternum the relation of the sternoclavicular articulation to the 

 large vessels and to the contents of the superior aperture of the thorax (see Plates n, 13, and 16) 

 must be borne in mind, particularly with reference to the backward dislocation of the sternal end 

 of the clavicle. In this dislocation the sternal end of the clavicle may press backward upon the 

 subclavian artery and vein, the common carotid artery, the internal jugular vein, the trachea, and 

 the esophagus, and lead not only to compression of the vessels but also to dyspnea and dysphagia. 



The Lateral Thoracic Wall. The mammary gland of the male is practically devoid of 

 interest. The mammary gland of the female lies upon the pectoralis major between the third 

 and seventh ribs and possesses from fifteen to twenty secretory ducts which converge toward 

 the nipple and have dendritic ramifications. In opening mammary abscesses (after mastitis) 

 the incisions should consequently be made to radiate from the nipple in order to avoid injuring 

 these ducts. 



The arteries oj the mammary gland reach their most marked development during the height 

 of lactation. They originate from three sources: (i) From the perforating branches of the in- 

 ternal mammary artery in the upper five intercostal spaces, but particularly in the second and 



