EXTERNAL AND L\TER.\AL IXTERCOSTALS 395 



THE MUSCLES OF THE THORAX 



These consist of six muscles, or sets of muscles, which are attached chiefly to 

 the ribs, their cartilages, and the sternum: viz. the external and internal inter- 

 costals, thelevatores costarum, the triangularis sterni, infracostales (or subcostales), 

 and diaphragm. 



Intercostal Muscles 



The intercostal muscles — named from their position — are long narrow sheets 

 of short oblique muscuhvr filjres Avhich occupy the intercostal spaces. The fibres 

 of the outer sheet run downwards and forwards, and those of the inner downwards 

 and backwards; and the two sheets are the upper continuations of the obliquus 

 extenius and internus of the abdominal Avail. 



1. THE EXTERNAL INTERCOSTALS 



The external intercostals, AA'hich are stronger than the internal, are eleven in 

 number, and till the spaces between the ribs from the tubercle to the tip. In the 

 higher spaces, however, they do not come quite so far forwards as below. Above, 

 the lower attachment barely reaches the tij) of the rib; while below, the upper 

 attachment reaches the tip, and the lower is upon the cartilage. 



Origin. — The lower or outer border of all the ribs except the last, from tubercle 

 to anterior extremity. 



Insertion. — The outer aspect of the upper border of all the ribs but the first 

 from a little in front of the tubercle to the tip, or in the lower ribs for a short 

 distance upon the cartilage. 



Structure. — Composed of obliquely directed parallel bundles of fleshy fibre, 

 with a short tendinous origin, and with a slight admixture of fibrous tissue. The 

 posterior are more oblique than the anterior fibres. The sheet formed by their 

 fibres is thickest behind, and becomes gradually thinner forwards. Between the 

 cartilages it is succeeded by a thin memljrane, the external intercostal fascia, 

 which is composed of fibres running with the same slope as those of the muscle. 



Nerve-supply. — The intercostal nerves as they run forwards give numerous 

 filaments to the inner surfaces of the muscles. 



Action. — See later. 



Relations. — Superficially, the pectoralis major and minor, the serratus magnus, 

 the external oblique, the latissimus dorsi, the trapezius, rhomboidei, the serrati 

 postici, the continuation upAvards of the erector spinae, and the levatores costarum ; 

 deeply, the internal intercostals and infracostales, the intercostal A^essels and nerA'es. 



2. THE INTERNAL INTERCOSTALS 



The internal intercostals are eleven in number, and fill the spaces from the 

 angles of the ribs to the anterior extremities of the cartilages. The filn-es, Avhich 

 are shorter and not quite so oblique as those of the outer slieet, are directed doAvn- 

 Avards and backAvards. 



Origin. — The upper border of the subcostal groove of the eleven upper ribs 

 from the angle forAvards, and the continuation of this border upon the cartilages. 



Insertion. — The inner asjiect of the upper border of the eleven lower rilts and 

 cartilages. 



Structure. — The sheets are thicker in front. There is less fibrous tissue mixed 

 Avith the flesliy fibres than in the outer sheet. In the upper and loAver spaces the 

 fleshy fibres are continued a little finther Ijack than the angles. The rest of the 



