1288 PHYSIOLOGY 



As soon as the os uteri 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 contractions 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 re- 

 newal of uterine contractions about twenty to thirty minutes after the 

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

 membranes. 



NERVOUS MECHANISM. We possess little experimental knowledge of 

 the nervous mechanism of parturition. The most important observation 

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

 physiologist observed the normal performance of menstruation (heat), 

 impregnation, and parturition in a bitch whose spinal cord had been com- 

 pletely divided in the dorsal region during the previous year. On the other 

 hand, destruction of the lumbo-sacral 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 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. 



