DETERMINING CAUSES. 263 



upon observation of the phenomena of pregnancy, it has appeared 

 to be more satisfactory than any other. It is said — if the cavity of 

 the body of the womb, only, enlarges during the four or five first 

 months, and that of the cervix afterwards dilates by degrees, frorn 

 above downwards, and confounds itself with the former, it depends 

 upon the circumstance that the fibres of the body and fundus, placed 

 lengthwise, and being the softest and most extensible, distend and 

 yield more easily than those of the neck, which are circular, denser, 

 more compact, and situated transversely. Between them is estab- 

 lished a kind of balancing or contest, which results in the induction 

 of labor: those of the body must be looked upon as so many loops 

 which embrace the ovum in their concavity, while their extremities 

 are attached to different points of the circles of the neck; the former 

 at first yield without difficulty, and even without re-acting upon the 

 latter; but about the middle of pregnancy, by elongating, they stretch 

 the fibres of the neck, whose circles disappear or are thus drawn in 

 succession into the body of the organ; so that at last the canal of 

 the neck no longer exists, but merely an orifice with a circumference 

 of greater or less thickness. There is then an equilibrium between 

 the neck and body of the womb; but as the looped fibres have now 

 no other resistance to overcome but the circular fibres, they triumph 

 over the os uteri with great facility, the equilibrium is soon broken, 

 and labor commences. 



670. According to this view of the circumstances, I should define 

 the determining cause of labor to be, the tendency of the fibres of 

 the body of the uterus to contract; a tendency or effort which pro- 

 duces no real and sensible effect, until from the moment when the 

 cervix ceases to furnish any further materials to the enlargement of 

 the womb. 



671. A. Petit expressed himself upon this subject somewhat dif- 

 ferently: " It cannot be doubted," says that author, " that the deter- 

 mining cause of the uterine contractions is the irritation experienced 

 by the uterus when pregnancy has reached its full term. I consider 

 the cervix as a magazine in which nature has placed in reserve the 

 quantity of muscular fibres which she needs, to furnish by their de- 

 velopment materials for the expansion of the womb during the course 

 of gestation. In the natural state this expansion, when once begun, 

 proceeds pari passu with the growth of the foetus; every thing is 

 proportioned, fixed, so that when the latter is sufficiently developed 

 to bear the action of external agents, and employ them for its own 

 benefit, all the fibres of the cervix have yielded, and the magazine 

 is exhausted; labor will therefore take place when all the fibres that 

 had been placed in reserve in different parts of the womb, and chiefly 



