Muscles of the Cliest. 267 



M. serratus anterior (0. T. serratus magnus) (see Figs. 308, 310, 311, 31(;, 317 

 and 343). Form: flat, quadrangular. Position: on the lateral and posterior wall of the thorax, 

 close to the ribs and mm. intercostales , covered in front and al)Ove by the ram. pectorales, 

 behind by the m. subscapularis , by the scapula and its posterior muscles as well as by the 

 m. latissiraus dorsi; lateralward and below, with its 4 5 inferior digitations, just beneath the 

 skin. Origin: by 910 tieshy digitations from the 89 uppermost ribs (by 2 digitations 

 from the 2ndrib); also from a tendinous arch between 1^' and '^^jj^- Insertion: uppermost 

 2 digitations (from 1*^ and 2"^ rib) converging to the angulus medialis scapulae; 2^^ digitation 

 (from 2iid rib) runs diverging to the margo vertebralis: 4"i 10<i digitations (from 3id gth 

 rib) extend converging to the angulus inferior; the latter portion is the strongest. The 

 fibers run as a whole curved so as to be convex and to follow exactly the curvature of the 

 wall of the thorax. Action: it presses the scapula to the thorax, draws it forward and lateral- 

 ward, acting with especial power upon the angulus inferior; at the same time it rotates the 

 scapula so that the angulus lateralis is elevated. Innervation: n. thoracalis longus. 



Mm. intercostales externi (see also Figs. 311, 319 and 33G). Form: flat, short, 

 thin. Position: in the intercostal spaces, from the tubercles of the ribs on, to about the lateral 

 ends of the costal cartilages; behind they are continous with the mm. levatores costarura. 

 Origin: lower margin of each rib, external fnim the sulcus costae. Insertion: the fibers run 

 parallel downward and st(n-nalward to the upper margin of the next rib below. Medianward 

 from the anterior end running in the same direction are glistening tendinous bands of connective 

 tissue (ligamcnta intercostalia externa, not shown in the figure). Action: muscles of inspi- 

 ration. Innervation: rami anteriores [nn. intercostales] of the nn. thoracales I XI. 



Mm. intercostales iiiterui (see also Figs. 310. 311, 313 and 321). Form: flat, short, 

 thin. Position: in the intercostal spaces from the region of the angles of the ribs to the 

 sternum: they lie more internally than the mm. intercostales externi, and are accordingly 

 covered by the latter externally as far as the lateral ends of the costal cartilages; between 

 the costal cartilages they are covered only by the ligg. intercostalia externa; behind they 

 leave the anterior surface of the nun. intercostales externi free medianward from the angles of 

 the ribs. Origin: inferior margin of each rib, internally from the sulcus costae. Insertion: 

 the fibers run parallel doT\'nward and vertebralward to the upper margin of the next ril) 

 below. Action: the portions extending between the ossa costalia are muscles of exspiration : 

 those bundles running between the costal cartilages, mm. mtercariilaf/iiiei , act like the 

 mm. intercostales externi. Innervation: rami anteriores [nn. intercostales] of the nn. 

 thoracales I XL 



The ligamenia intercostalia interna are thin bands of connective tissue which are 

 situated, partly in front on the inner side of the mm. intercostales interni, in connection with 

 the m. transversus thoracis (not shown in figure), partly behind on the anterior surface 

 of the mm. intercostales externi in connection with the nun. intercostales intemi and the 

 mm. subeostales. 



Mm. subcostales, very variable. Form: flat, triangular, very thin. Position: on the 

 internal surface of the ribs, usually only of the lower ribs, in the region of the anguli costarum. 

 Origin: frum the inner surface nf the ribs. Insertion: the fibers run like those of the mm. 

 intercostales interni, but jump over one or two ribs. Action: muscles of expiration. Inner- 

 vation: rami anteriores [nn. intercostales] of the corresponding nn. thoracales. 



M. transversus thoracis (see Fig. 313). Form: flat, quadrangular, very thin. 

 Position: directly upon the posterior surface of the anterior thoracic wall, below close to the 

 diaphragm and bounding the m. transversus abdominis. Origin: posterior surface of the coi-pus 

 sterni and of the proc. xiphoideus. Insertion: obliquely upward and lateralward to the 3"i 

 (or also 2ndj_6th rib. Action: it draws the ribs downward on expiration. Innervation: 

 rami anteriores [nn. intercostales] of the nn. thoracales III (or II) VI. 



