186 MUSCLES OF THE THORAX. 



Muscles of the Thorax. 



The principal muscles situated upon the thorax belong in their 

 actions to the upper extremity, with which they will be described. 

 They are the pectoralis major and minor, subclavius and serratus 

 magnus. The true thoracic muscles are few in number, and apper- 

 tain exclusively to the actions of the ribs ; they are, the 



Intercostales externi, 

 Intercostales interni, 

 Triangularis sterni. 



The intercostal muscles are two planes of muscular and tendinous 

 fibres directed obliquely between the adjacent ribs and closing the 

 intercostal spaces. They are seen partially upon the removal of 

 the pectoral muscles, or upon the inner surface of the chest. The 

 triangularis sterni is within the chest, and requires the removal of the 

 anterior part of the thorax to bring it into view. 



The intercostales externi, eleven on each side, commence pos- 

 teriorly at the vertebral column, and advance forwards to the costal 

 cartilages where they terminate in a thin aponeurosis which is con- 

 tinued onwards to the sternum. Their fibres are directed obliquely 

 downwards and inwards, pursuing the same line with those of the 

 external oblique muscle of the abdomen. They are thicker than 

 the internal intercostals. 



The intercostales interni, also eleven on each side, commence 

 anteriorly at the sternum, and extend backwards as far as the angles 

 of the ribs, whence they are prolonged to the vertebral column by a 

 thin aponeurosis. Their fibres are directed obliquely downwards 

 and backwards, and correspond in direction with those of the inter- 

 nal oblique muscle of the abdomen. The two muscles cross each 

 other in the direction of their fibres. 



In structure the intercostal muscles consist of an admixture of 

 muscular and tendinous fibres. They arise from the two lips of the 

 lower border of the ribs, the external from the outer lip, the internal 

 from the inner, and are inserted into the upper border. 



Relations. The external intercostals, by their external surface 

 with the muscles \vhich immediately invest the chest, viz. with the 

 pectoralis major and minor, the serratus magnus, serratus posticus 

 superior and inferior, scalenus posticus; sacro-lumbalis, and lon- 

 gissimus dorsi, with their continuations, the cervicalis ascendens and 

 transversalis colli ; the levatores costarum, and the obliquus externus 

 abdominis. By their internal surface with the internal intercostals, 

 the intercostal vessels and nerves, and a thin aponeurosis, and pos- 

 teriorly with the pleura. The internal intercostals, by their external 

 surface with the external intercostals, and intercostal vessels and 

 nerves ; by their internal surface with the pleura costalis, the trian- 

 gularis sterni and diaphragm. 



Connected with the internal intercostals are a variable number of 



