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ANIMAL MECHANICS. 



muscular fibres, when combined with the corresponding fibre 

 of the other side, produces a vertical strain acting from P to- 

 wards 8, and a horizontal strain which is equilibrated by the 

 equal and opposite strain on the other side of the abdomen ; — 

 and there is a remarkable difference between these two tensile 

 strains, for the transverse strain is constant along the entire 

 linea alba, while the vertical strain is greatest at P, and di- 

 minishes from P to 8. Take, for example, the fibres NX, 

 passing through the navel ; the muscular fibres between NX 

 and B, produce along the line PN vertical components acting 

 towards N, and consequently they produce no strain whatever 

 at the point N; on the contrary, the fibres lying between 

 NX and A, all combine to produce a vertical strain at N, 

 acting towards the point 8. It is therefore evident that the 

 vertical strain on the linea alba caused by the external oblique 

 muscle, increases from 8 to P, and varies in the proportion 

 of the distance of the point considered from 8. The compo- 

 nents of the external oblique muscle, acting upon a linear inch 

 at the navel, may now be calculated. 



Draw the line NX, and let fall the perpendicular AK 

 upon it. If t be the thickness of the muscle, and $ the angle 

 XN8, we have for the vertical component, 



V = AK x t x 2 cos $ ; 



and for the horizontal component, 



H = AK x t x sin 0. 



In order to find the vertical tensile strain, on a linear inch, 

 we must take account of the width of the recti abdominis 

 muscles, for as the fibres terminate in the line AB, at the outer 

 border of the recti muscles, their vertical component is to be re- 

 garded as distributed along the entire breadth of those muscles. 



