$66 THE PHYSIOLOGY OF REPRODUCTION 



At the commencement of labour the contractions do not occur 

 oftener than once every half or quarter of an hour, but they soon 

 become more frequent, and recur eventually at intervals of two or 

 three minutes. Their average duration is about a minute, though 

 actual pain is experienced for a shorter time.^ Polaillon ^ and 

 Schutz ^ have shown from tracings that the period of increase occupies 

 the main portion of the " pain," its acme being of short duration. In 

 animals possessing bi-cornuate uteri the contractions are said to be 

 peristaltic in nature, but this is not so evident in the case of the 

 hiunan subject. 



Williams * has discussed the question as to the amount of force 

 exerted at each " pain " in a woman during delivery. He states that 

 the expenditure of energy necessary to restrain the head of the child 

 as it emerges from the vulva is represented by not more than fifty 

 pounds, although the obstetrician sometimes finds it impossible to 

 hold it back at the acme of the pain. Schutz ^ made an attempt to 

 arrive at a more exact estimation by inserting into the uterus a 

 rubber bag connected with a mercury manometer. He found that 

 whereas the intra-uterine pressure^ between the contractions was 

 represented by a column of mercury of 20 mm., during the pains it 

 rose to a height of from 80 to 250 mm. This difference is calculated 

 to represent a force of from 8^- to 27^ pounds. 



The clinical course of labour and the muscular forces concerned 

 in the process are fully dealt with in the text-books on Midwifery,'' 

 and it is not proposed in the present work to devote more than a 

 very brief space to the consideration of this subject. It is customary 

 to divide the period of labour into three stages. 



The first stage is characterised by the dilatation of the cervix 

 and OS uteri. Galabin gives the following account of the mechanical 

 processes which take place in, the uterus during this stage of labour : 

 "There are three elements in the mechanism of dilatation of the 

 cervix and os; first, the mechanical stretching by the bag of mem- 

 branes; secondly, the contraction of the longitudinal fibres of the 

 uterus, which draw the cervix open ; and thirdly, the physiological 

 relaxation of the circular fibres, which [is always associated] with the 

 contraction of the body of the uterus. It follows from the principles 



1 Williams (W.), Obstetrics, London, 1904. 



'■^ Polaillon, Recherches sur la Physiologie de I' Uterus Oravide, Paris, 1880. 



■' Schutz, " tJeber die Formen der Wehenerven und iiber die Peristaltik des 

 Mensohlichen Uterus," Arch.f. Cfynalc., vol. zxvii., 1886. 



* Williams,, foe. cit. 



° Schutz, " Ueber die Entwickelung der BJuft des Uterus in Verlaufe der 

 Geburt," Verhandl d. Deutsch. Gesell. fur Gyndk., 1895. 



^ See Williams, loc. cit. Galabin, Manual of Midwifery, 6th Edition, 

 London, 1904, and the other text-books on the subject. See also Sellheim, 

 "Die Physiologie der Weiblichen Geschlechtsorgane," Nagel's Handbuch der 

 Physiologic des Menschen, vol. ii., Braunschweig, 1906. 



