228 NORMAL PARTURITION. 



SECTION I.— CAUSES OF PARTURITION. 



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 

 physiology of the generative organs. During the evolution and deve- 

 lopment of the ovum, the uterus increases in a corresponding manner, and 

 its muscular structure 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 com- 

 pletely 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 materials between the uterus and foetus is lessened, and the latter — 

 always more or less of a parasite — becomes like a foreign body in the 

 cavity of the former, its greatly augmented weight and volume also 

 aiding in the change. 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 contraction of the uterine 

 muscles, as Schroeder observes, a certain amount of continuous irrita- 

 tion is necessary. This sum once obtained, a reflex action takes place 

 in the form of 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 contrac- 

 tions the uterine wall is removed from the envelopes, and this separa- 

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

 reflex action, in the form of labour-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 labour, as well as the not infrequent retardation of the 

 pains in cases where the separation of the membranes has been pre- 

 mature. 



The sympathetic nerve is in all probability that which is most con- 

 cerned 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 produced 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 I [.—THE EXPELLING POWERS. 



The expulsive force by which parturition is effected resides in the 

 unstriped muscular fibres of the uterus ; these cause the organ to con- 

 tract in a rhythmical and somewhat peristaltic manner, the contrac- 

 tions of the abdominal muscles and diaphragm being merely auxiliary. 

 The contractions of the uterus may take place although the organ 



