THE THORAX 



Thoracic Wall. 



Muscles — Intercostal Muscles. — These are two in number in each 

 space, external and internal, and they are arranged as thin sheets 

 of obliquely-disposed muscular fibres, with a large admixture of 

 tendinous fibres. 



External Intercostal Muscles — Origin. — ^The lower border of the 

 upper rib. 



Insertion.- -The outer margin of the upper border of the lower rib. 



. Internal Mammary 

 Artery 



External Intercostal Muscle 



Interosseous Part of Internal 

 Intercostal Muscle 



Interchondral Part 



of Interi'al 

 Intercostal Muscle 



External Intercostal Muscle 



Fig. 380. — Diagram of the Intercostal Muscles and Intercostal 



Arteries 



(A small portion of the external intercostal muscle has been divided and 



reflected). 



Nerve-supply.- -The intercostal nerve of the corresponding space. 



The fibres of each muscle are directed downwards and forwards, 

 and in this respect coincide with those of the obliquus externus 

 abdominis. The muscles, as a rule, extend as far forwards as the 

 junction of the ribs with their cartilages, but sometimes they stop 

 short of this point, especially in the upper spaces. From the 



920 



