248 PHYSIOLOGY CHAP. 



Since uterine innervation is not clearly understood, we are 

 not as yet in a position to form an exact opinion as to the causes 

 which bring about labour at the end of the fortieth week of 

 pregnancy. It is certain that as the gravid uterus increases in 

 size, the excitability of the uterine muscles is raised proportion- 

 ately. During the last months and weeks of pregnancy the slight 

 pains which mark a gradual transition from the painless con- 

 tractions of gestation to the intense and painful contractions of 

 labour occur more and more frequently. The hypertrophy of the 

 muscular fibres and their steadily increasing mechanical com- 

 pression probably cause the heightened nervous and muscular 

 excitability. The progressive increase of pressure exerted by 

 the foetal head on the lower part of the uterus, which is more 

 liberally supplied with nerves, undoubtedly helps to maintain 

 the contractile activity of the uterus so long as its contents have 

 not been expelled. Another cause of parturition has been thought 

 to be a progressive fatty degeneration of the decidua, which renders 

 the relation between the ovum and the uterus less and less 

 intimate, and at last makes the foetus an extraneous body as 

 regards the mother. Finally, it cannot be denied that during 

 the lying-in period the uterine contractions, which occur with 

 increasing frequency, interfere more and more seriously with 

 the gaseous exchange between the mother and the foetus ; this 

 gives rise to greater venosity of the placental blood, which in 

 turn increases the excitability of the ganglia of the uterine 

 walls, and causes the expulsive contractions and supreme pains 

 which precede parturition. 



It was formerly thought that the obliteration of the placental 

 vessels was one of the internal conditions determining parturition ; 

 Chappentier (1889) observes, when writing on this subject, that 

 the real cause of parturition must be sought in the vascular 

 relations between the ovum and the uterus. 



"The more the ovum is developed, the more will the intra- 

 uterine pressure increase, causing an equivalent reaction of the 

 uterine walls. The eccentric tension is counterbalanced by the 

 concentric uterine reaction of the walls of the uterus. This 

 contest, which is much more acute during the last period of 

 pregnancy, gives rise to a secondary phenomenon, influenced by 

 the increasingly intense uterine contractions : the closure of the 

 vessels, which takes place at every contraction. At last the con- 

 nection between the uterus and the placenta is so constantly 

 interrupted that an intermediate zone is created in the thickness 

 of the decidua between the ovum and the uterus, the zone of 

 rupture of Langhans, Dohrn and Sinety, in which the degenerate 

 and fatty elements, and the partially or wholly obliterated vessels 

 (Leopold) form a tissue devoid of resistance or vitality. 



" From this moment, moreover, the ovum, having an inadequate 



