SERRATUS MAGNUS 



299 



Insertion. — The first part is attached to an oval space upon the venter of the 

 scapula close to the posterior superior angle. The second part, to tin- whole of 

 the vertehral l:)or(ler of the scai)iila upon its ventral aspect. The third part to the 

 large oval space on the venter of the scapula close to its inferior angle. 



Structure. — The origin of the muscle is by fleshy or shoit aponeurotic fibres, 

 and in the first part these fibres converge very slightly towards their fleshy inser- 

 tion. In the second part they diverge and form a thin sheet attached to the verte- 

 ])ral border of the scapula. In the third part of the muscle the fibres converge 

 fanwise and form a very thick and strong fleshy mass Avhich is inserted directly 

 into the inferior angle of the scapula. At their origin the teeth of this part of the 

 muscle interdigitate with or fit in l)etween those of the origin of the oljlicjuus 

 alxlominis externus. All the fibres are curved to adapt themselves to the convex 

 wall of tlie chest. 



Nerve-supply. — From the brachial plexus by the posterior thoracic nerve, 



Fig. 267. — Serratl's Magxls. 



Tipper part of 

 serratua tnaguus 



Middle part 



Lower part 



which is derived from the fifth, sixth, and seventh cervical nerves. After running 

 down the side of the chest upon the outer surface of the muscle, the nerve is dis- 

 tri1)uted by many branches to the various digitations. 



Action. — By its contraction, this muscle draws forwards the vertebral border of 

 the scapula and, as the third part of it is much the strongest, it will act especially 

 upon the inferior angle, and will rotate tlie scapula so as to raise the point of the 

 shoulder. It will therefore help the trapezius muscle in raising the shoulder, 

 and it will l)e l)rought powerfulh' into play wlienever the shoulder is used in push- 

 ing in a forward direction. It is most important, however, in relation to the 

 movements of. the arm. In order that the deltoid muscle may raise the humerus, 

 it is necessarv that the fulcrum formed by the glenoid portion of the scapula should 

 be held steady. For this reason the scapula extends so far downwards in order 

 that the leverage given to the third and most jjowerful portion of the serratus may 

 be as great as possible. When the serratus magnus is paralysed, all the efforts of 



