TBE THORAX 9^9 



cutaneous nerve, piercing in succession the following structures : 

 the interchondral portion of the internal intercostal muscle ; the 

 anterior intercostal aponeurosis ; the pectoralis major muscle ; and 

 the deep fascia. 



Branches. — ^These are muscular, lateral cutaneous, and anterior 

 cutaneous. 



The muscular branches supply the following muscles : the levatores 

 costarum, serratus posticus superior, subcostal muscles, intercostal 

 muscles, and triangularis stemi. 



The lateral cutaneous nerves arise about midway between the 

 vertebral column and the side of the sternum. They pierce the 

 external intercostal muscles, and make their appearance between 

 the digitations of the serratus magnus. The first intercostal nerve, 

 as a rule, gives off no lateral cutaneous branch, and that of the 

 second is known as the intercosto-humeral ner\'e. 



The anterior cutaneous nerves are the terminal branches of the 

 upper six intercostals, that of the first being sometimes absent. 

 For their description, see Index. 



Intercostal Arteries. — These vessels are arranged in two sets — 

 posterior and anterior. 



Posterior Series. — The intercostal arteries of the first two spaces 

 are derived from, the superior intercostal artery (see Index). The 

 intercostal arteries of the lower nine spaces are branches of the 

 descending thoracic aorta, and are called the aortic intercostal 

 arteries. Each at first lies between the posterior intercostal 

 aponeurosis and the parietal pleura. About the level of the angle 

 of the rib it pierces the posterior intercostal aponeurosis, and gives 

 off its collateral intercostal branch, which inclines downwards. 

 These two arteries now pass forwards between the two intercostal 

 muscles, the main aortic intercostal lying in the subcostal groove 

 of the upper rib, where it has the intercostal vein above it and 

 the intercostal nerve below it, and the collateral intercostal lying 

 along the upper border of the lower rib. These two vessels, in the 

 case of the upper nine spaces, terminate by anastomosing with the 

 anterior intercostal arteries, which are two in number in each of 

 these spaces, and will be presently described. These anastomoses 

 take place a little in front of the mid-point between the vertebral 

 Column and the side of the sternum. 



Branches. — These are dorsal, collateral intercostal, and lateral 

 cutaneous. 



The dorsal branch passes backwards to the muscles and integument 

 of the back, giving off in its course a spinal branch, which enters 

 the spinal canal through the intervertebral foramen. 



The collateral intercostal branch, as stated, arises about the level 

 of the angle of the rib, and inclines dowTiwards to the upper border 

 of the lower rib, along which it passes between the intercostal 

 muscles. 



The lateral cutaneous branch accompanies the corresponding lateral 

 cutaneous nerve. 



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