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210 MUSCLES OF THE THORAX. 



The actions of the remaining muscles of the spine, the supra and 

 inter-spinales and inter-transversales, are expressed in their names. 

 They approximate their attachments and assist the more powerful 

 muscles in preserving the erect position of the body. 



The levatores costarum raise the posterior parts of the ribs, and are 

 probably more serviceable in preserving the articulation of the ribs from 

 dislocation, than in raising them in inspiration. 



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 appertain 

 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 in- 

 tercostal spaces. They are seen partially upon the removal of the pec- 

 toral 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 tubercles of the ribs, and advance forwards to the cos- 

 tal 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 ex- 

 ternal oblique muscle of the abdomen. They are thicker than the in- 

 ternal 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 internal ob- 

 lique 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 which immediately invest the chest, viz. the pecto- 

 ralis major and minor, the serratus magnus, serratus posticus superior 



