ANIMAL MECHANICS. 



219 



Of these muscles, the rectus abdominis produces a tensile 

 strain altogether in the vertical direction, and the transversa- 

 lis causes a tension in the horizontal direction only. These 

 two tensions are nearly in the proportion of 2 to 1, which ap- 

 proximates the ratio of the diameters of curvature. If there- 

 fore these muscles, alone, were in action, the theory would 

 be fully vindicated. The oblique muscles, however, produce, 

 each of them, a vertical and horizontal component strain at 

 the navel, which must be added to the former, to determine 

 the total strain acting in the walls of the abdomen in those 

 directions. Let us now proceed to calculate these compo- 

 nents, and test the entire the ory. 



Let 0, E(Fig. 52), re- 

 present the obliquus externus 

 muscle, and SNP the linea 

 alba, S denoting the end of 

 the sternum, N the navel, 

 and P the pubes ; — II. being 

 the crest of the ilium. The 

 line AB represents the inner 

 limit of the muscular fibres, 

 where they become inserted 

 into the fascial sheet enclos- 

 ing the rectus abdominis. 

 The fibres of the external 

 oblique muscle are approx- 

 imately parallel to each 

 other, and constitute & pris- 

 matic muscle spread in a 

 sheet over a convex sur- 

 face, — and the prolonga- 

 tions of its fibres to the 

 linea alba make angles of 35 0 with that line. Each of the 



Fig. 52. 



