PHYSIOLOGY OF PARTURITION. 



SECTION I. — CAUSES OF PARTURITION. 



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Though parturition only occurs, at the end of pregnancy, nevertheless 

 this act is being prepared for from an early period in the development of 

 the ovum, as we have shown when speaking of the anatomy and physiol- 

 ogy of the generative organs. During the evolution and development of 

 the ovum, the uterus increases in a corresponding manner, and its mus- 

 cular layer is proportionately augmented. When, towards the termination 

 of gestation, the ovum has reached maturity, and the organs necessary for 

 the independent existence of the foetus are completely developed, certain 

 alterations occur, both in the uterus and the foetal connections with it, 

 which bring about the expulsion of the young creature. 



These alterations would appear to consist in a gradually increasing 

 fatty degeneration of the decidua, by which the organic connection that 

 had existed between the peripheral portions of the ovum and the uterus 

 is gradually destroyed, by a regressive process in the cells lying between 

 them ; while the blood which was sent to the organ is now diverted 

 towards the mammae, for the secretion of milk. The exchange of mate- 

 rials between the uterus and foetus is lessened, and the latter becomes like 

 a foreign body in the cavity of the former. At all the places where the 

 cell degeneration has reached a certain stage, the terminations of the 

 nerves are irritated. But to obtain a reflex action, and consequent con- 

 traction of the uterine muscles, as Schroeder observes, a certain amount 

 of continuous irritation is necessary. This sum once obtained, a reflex 

 action takes place in the form gf a contraction, which, however, is slight 

 at the beginning. Then a pause follows, until the sum of the irritation is 

 again sufficient to cause, a contraction. By the increase in intensity of 

 the contractions, the uterine wall is removed from the envelopes, and this 

 separation becomes a new source of irritation to the uterine nerve-fibres. 

 The reflex action, in the form of labor-pains, becomes more and more 

 powerful, until these follow at last in rapid succession and complete the 

 expulsion of the ovum. This irritability of the uterine nerves progres- 

 sively increases with the advance of pregnancy, and explains the regular 

 setting in of labor, as well as the not unfrequent retardation of the pains 

 in cases where the separation of the membranes has been premature. 



The sympathetic nerve is in all probability that which is most concerned 

 in the uterine contractions, as it is the motor nerve of the organ ; and its 

 influence is called forth by the irritation just mentioned as being pro- 

 duced on the terminations of the nerves on the inner surface of the uterus 

 by reflex action, the irritation being transformed into involuntary motor 

 activity. It is surmised that the sacral nerves are merely inhibitory. 



SECTION II. THE EXPELLING POWERS. 



The expulsive force by v/hich parturition is effected resides in the un- 

 striped muscular fibres of the uterus, which cause the organ to contract in 

 a rhythmical and somewhat peristaltic manner, the contractions of the 

 abdominal muscles and diaphragm being merely auxiliary. These con- 

 tractions of the uterus may take place although the organ does not con- 

 tain a foetus, and have been noted in extra-uterine pregnancy, when they 

 probably occur through sympathy. They have been observed in the false 

 gestation of the Bitch which has not really conceived, but whose mammae 

 enlarge, and which makes its bed and exhibits other indications of ap- 

 proaching labor; us well as in pregnancies prolonged beyond their ordi- 

 narv limit. 



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