222 



ANIMAL MECHANICS. 



navel, and the lower fibres give a vertical component which 

 draws the linea alba, upwards, towards the navel ; hence» 



neither sets of fibres 

 can produce any tensile 

 strain at the navel itself. 

 The only portion of the 

 muscle that causes a 

 vertical tension at the 

 navel, is the portion 

 contained between the 

 horizontal fibres X Y 

 and the line NX, which 

 is the direction of the 

 "* fibres passing through 

 the navel itself. This 

 portion of the muscle, 

 and the corresponding 

 portion of the similar 

 muscle, at the other 

 side, will behave as a tri- 

 angular muscle, whose 

 resultant ZX bisects the 



angle NXY, and is equal to (p. 181) 

 2 / sin 0, 



where 0 denotes half the angle NXY, and /denotes the cross 

 section of the muscle. Hence the vertical resultant of the in- 

 ternal oblique muscle at both sides is 



2 /sin 0 x 2 cos 0, 



where <p denotes the angle made with the linea alba by the 

 line ZX, and is the complement of 0. This resultant must be 

 divided by the total width of the recti muscles, in order to find 



