532 THE PHYSIOLOGY OF REPRODUCTION 



ment is still further increased by the growth of the uterine 

 wall itself. Partly as a consequence of this enlargement the 

 waves of contraction which were -present at the beginning of 

 pregnancy, or even previously, as above described, become 

 much more marked, but are stiU unaccompanied by painful 

 sensation. With the onset of labour, however, these uncon- 

 scious painless contractions are replaced by others of increasing 

 intensity, and in the human subject distinctly affecting con- 

 sciousness and giving rise to severe suffering. These are the 

 " labour pains " which bring about the dilatation of the cervix 

 uteri and lead to the expulsion of the child followed by the 

 placenta. 



At the commencernent 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 in- 

 tervals 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 human subject. 



WiUiams * has discussed the question as to the amoimt of 

 force exerted at each " pain " in a woman during dehvery. 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 some- 

 times 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 



' Williams, Obstetrics, London, 1904. 



^ Polaillon, Recherches sur la Physiologic de l' Uterus Gravide, Paris, 

 1880. 



' Schatz, " Ueber die Formen der Weheuerven und iiber die Peristaltik 

 des Menschlichen Uterus," Arch. f. Oynak. , vol. xxvii., 1886. 



* Williams, loc. cif. 



<> Schutz, " Ueber die Entwickelung der Kraft des Uterus in Verlaufe der 

 Geburt," Verhandl. d. Deutsch. Oesell./iir Oynak., 1895. 



