THE THORACIC WALL 503 



this account, there is little danger of injury to the deeper 

 structures. 



The Intercostal Spaces are occupied by the internal and 

 external intercostal muscles, with their vessels and nerves. The 

 fibres of the external intercostals pass downwards and medially 

 from the lower border of the costal groove of the rib above to 

 the upper border of the rib below. The fibres of the internal 

 intercostals pass downwards and laterally from the upper 

 border of the costal groove of the rib above to the upper border 

 of the rib below. 



The costal groove is placed on the lower part of the inner 

 surface of the rib and is best marked posteriorly. It lodges 

 an intercostal vein, artery, and nerve, which lie in that order 

 from above downwards so that the nerve is more exposed than 

 the vessels. On this account, when a needle is passed into 

 the chest, the instrument is kept close to the upper border of 

 a rib. 



Medial to the costo - chondral junctions the external 

 intercostal muscle becomes a fibro - tendinous sheet, which 

 is termed the anterior intercostal membrane. The internal 

 intercostal muscle, although extending to the lateral margin 

 of the sternum in front, is deficient at the vertebral end of the 

 intercostal space, where it is replaced by the posterior intercostal 

 membrane. 



The lateral extremities of the costal cartilages become 

 enlarged in rickets and form small elevations, which constitute 

 the condition known as the " rickety rosary." 



The Internal Mammary Artery descends at a distance of 

 half an inch from the side of the sternum. Posteriorly, it rests 

 on the costal pleura and, in the lower part of its course, on 

 the slips of the transversus thoracis (triangularis sterni), which 

 pass obliquely from the posterior aspect of the sternum to the 

 upper costal cartilages. Anteriorly, the artery is in relation 

 to the costal cartilages and the internal intercostal muscles, 

 and is crossed by the intercostal nerves. At the sixth inter- 

 costal space the internal mammary artery ends by dividing 

 into the superior epigastric (p. 253), which descends in the 

 sheath of the rectus abdominis, and the musculo-phrenic, which 

 runs downwards and laterally along the costal origin of the 

 diaphragm. 



Ligature of the internal mammary vessels for stab-wounds 

 should be carried out through a transverse incision over the 



32 c 



