The Thorax. 117 



their substance almost as far as the cartilages, near which, 

 they pass beneath the internal muscles, and then, piercing 

 them close to the sternum, become cutaneous. 6. The inter- 

 costal vessels pass diagonally across the parallelogram 

 bounded by the ribs, above and below; internally, by the 

 lamina of the vertebrae, and externally, by a line joining the 

 angles of the ribs. After giving off branches to the ribs be- 

 low, they continue onwards in the subcostal grooves on the 

 lower borders of the ribs above, and anastomose with in- 

 tercostal branches from the internal mammary artery. 

 7. The internal mammary is a branch of the first part of the 

 subclavian and descends behind the costal cartilages as far 

 as the sixth, where it divides into the musculo-phrenic and 

 the superior epigastric arteries. 8. The Sternum. In 

 the middle line, under the skin and superficial fascia, is 

 the sternum, consisting of the manubrium, the gladiolus, 

 and the ensiform cartilage. It forms the bottom of the 

 sternal groove between the pectoral muscles. Above the 

 sternum is the suprasternal notch, while below, is a de- 

 pression called the subcostal angle, in the centre of which 

 the ensiform cartilage projects. The sternum, with its 

 costal cartilages, is directed obliquely downwards and for- 

 wards, so as to form an angle of 25 or 30 with the verti- 

 cal line. The sternum is developed from six centres, those 

 for the body or gladiolus being placed side by side, and 

 hence, arrested development of this part may result in the 

 production of a foramen or fissure between the lateral 

 centres. Lastly, two small bones, vestigial structures, 

 are occasionally found close to the upper border of the 

 sternum, and are called "episternal bones." 



Landmarks of the Thoracic Wall. The upper 

 border of the sternum is on a level with the intervertebral 

 disk between the second and third dorsal vertebrse. The 

 junction of the manubrium and gladiolus forms a promi- 



