NER VO US ME CHANISM. 3 5 1 



blood as the foetus becomes mature, and provoke, by their direct action 

 on the uterus or spinal cord, the uterine contractions which give rise to 

 labour. 



Actual parturition is in man generally divided into two stages. In 

 the first stage, the contractions are confined to the uterus, and chiefly 

 act in dilating the os uteri. In this dilatation two factors are in- 

 volved, namely, the active dilatation brought about by the con- 

 traction of the longitudinal muscular fibres which form the chief 

 constituent of the lower part of the uterine wall ; and, in the second 

 place, a passive dilatation by the pressure of the foetal bag of membranes, 

 which is filled with amniotic fluid, and forced down as a fluid wedge into 

 the os by the contractions of the uterine fundus. 



The uterine contractions are essentially rhythmical, being feeble at 

 first, and increasing gradually in intensity to a maximum which endures 

 a certain time, and then gradually subsides. The frequency and dura- 

 tion of the contractions increase as labour advances. 



As soon as the os is fully dilated and the foetal head has entered 

 the pelvis, the contractions change in character, being much more 

 prolonged and frequent, and attended by more or less voluntary con- 

 tractions of the abdominal muscles. This action of the abdominal 

 muscles is associated with fixation of the diaphragm and closure of the 

 glottis, so that pressure is brought to bear on the whole contents of the 

 abdomen, including the uterus. No expelling force can be ascribed to 

 the vagina, since it is too greatly stretched by the advancing foetus. 



In this way the foetus is gradually thrust through the pelvic canal, 

 dilating the soft parts which impede its progress, and is finally expelled 

 through the vulva, the head being born first. The membranes generally 

 rupture towards the end of the first stage of parturition. 



A third stage of labour is generally described. This consists in a 

 renewal of uterine contractions about twenty to thirty minutes after 

 the birth of the child, and results in the expulsion of the placenta and 

 foetal membranes. 



Nervous mechanism. — We possess little experimental knowledge of 

 the nervous mechanism of parturition. The most important observa- 

 tion on this point is the already quoted experiment by Goltz, 1 in which 

 this physiologist observed the normal performance of menstruation 

 (heat), impregnation, and parturition in a bitch whose spinal cord had 

 been completely divided in the dorsal region during the previous year. 

 On the other hand, destruction of the lumbar cord completely abolishes 

 the normal uterine contractions of parturition, so that this act must be 

 regarded as essentially reflex, presided over by a controlling ' centre ' 

 in the grey matter of the lumbar cord. 



The activity of the centre can be inhibited or augmented by 

 impulses arriving at it from the peripheral parts of the body, as by the 

 stimulation of sensory nerves, or from the brain, as under the influence 

 of emotions. The nerve paths from the centre to the uterus have been 

 already described in the previous section. 



1 Arch. f. d. yes. Physiol., Bonn, 1874, Bd. ix. S. 552. 



