THE MUSCULATURE OF THE TRUNK. 295 



afford the proper protection to the vessels and nerves 

 therein contained as they pass from one lateral canal 

 to the next, in the interspaces between the vertebrae, 

 where, of course, no osseous canal affords protection. 



139. The triangularis sterni 1 is a beautifully deve- 

 loped muscle within the cavity of the thorax. 



It arises, somewhat tendinous, from the entire 

 superior margin of the summit of a costal process, 

 and its fibres directed backwards and upwards soon 

 divide into four well-defined digitations ; which latter 

 covering the inner surfaces of the leading three costal ribs, 

 become inserted into the first four as high up as their 

 articulations with the vertebral ribs. 



It will at once be seen that, the sternum being fixed, 

 a contraction of these muscles will result in drawing 

 down the costal ribs ; which, diminishing the cubical 

 contents of the thorax ; they thus become a powerful 

 auxiliary to the act of expiration. 



The triangularis sterni in man, although it has 

 a little different origin, fulfils precisely the same 

 function. 



1 According to Gadow, the following synonyms of the triangularis 

 sterni occur in literature (loo. cit., p. 125) : 



" 18/*. M. COSTI-STERNALIS. 



Der Erheber der Rippenfortsatze. Merrem. 



Der innere oder dreieckige. Brustrauskel. (M. sterno-costalis s. 



triangularis sterni} Tiedemann, 236. 

 Triangulaire du sternum. Cuvier. 



,, Gervais et Alix, p. 16. 



tSterno-costalis. Tiedemann. 

 Ohne Namen. Meckel, System, p. 502. 

 Triangularis sterni s. sterno-costalis. Magnus, p. 226. 

 Sterno-costal. Alix, p. 386. 

 Triangularis sterni. Selenka, p. 104. 

 Watson, p. 68." 



