1870.] 



Muscular Forces employed in Parturition. 



259 



If we suppose a ribbon, one inch in width, to be formed from the wall of 

 the uterus, its thickness will be 0* 1 51 9 inch ; and as each square inch of 

 cross section of muscular fibre is capable of lifting 102*55 lbs., we find for 

 the greatest tensile force producible by the contraction of the uterine 

 muscles : — 



Tensile strain of uterine | 102 -55 X 0-1519 = 15-577 lbs. 

 wall per inch J 



Substituting this value of T in the equation 



P = 2T 



9 



and for pits mean value 9*158 inches, we obtain the maximum hydrostatical 

 pressure inside the gravid uterus that can be produced by the contraction 

 of its muscular fibres : — 



Maximum hydrostatical pressure \ 2x 15*577 _ 3.402 \u 

 produced by uterine contraction J 9-158 



This pressure, applied to a circular surface of A\ inches in diameter, is 

 equal to 54*106 lbs. One hundred experiments were made by Duncan and 

 Tait upon the hydrostatical pressure necessary to rupture the membranes 

 which contain the liquor amnii, which are recorded in Dr. Duncan's book* 

 (pp. 306-311). The greatest pressure observed was 3*10 lbs., and the 

 least was 0*26 lb. ; and I find that the mean rupturing pressure of all 

 their experiments was 1*2048 lb. 



Combining this experimental result with the calculation already given, 

 of the amount of pressure producible by the muscular tissue of the womb, 

 we may conclude that the uterine muscles are capable of rupturing the 

 membranes in every case, and possess, in general, nearly three times the 

 amount of force requisite for this purpose. 



In the second stage of labour, the voluntary action of the abdominal 

 muscles is called into play to aid the expulsive efforts of the uterine muscles. 

 I have attempted to calculate the force available from the contraction of 

 these muscles as follows. 



The abdominal muscles are four in number, viz. rectus abdominis, 

 obliquus externus, obliquus internus and transversalis. The last three 

 muscles form curved sheets, acting upon the corresponding muscles of the 

 opposite side by means of tendinous aponeuroses which meet in the linea 

 alba, and form the sheath of the vertical rectus abdominis muscle. From 

 the arrangement of all four, it is plain that the tensile force of muscular 

 contraction in the curved wall of the belly, from the xiphoid cartilage to the 

 symphysis pubis, is to be measured by the sum of the united forces of all 

 the muscular sheets. If we knew the force of each muscle, and the prin- 

 cipal curvatures of the belly in the middle line, we could calculate, by La- 

 grange's theorem, the hydrostatical pressure inside the abdominal cavity 

 and available to expel faeces, urine, or a foetus. 



* Eesearches in Obstetrics. Edinburgh, 1868. 



x2 



