Chap, ii.] PREGNANCY AND BIRTH. 1137 



been run. In the human subject the period of gestation 

 generally lasts from 275 to 280 days, i.e. about 40 weeks, 

 the general custom being to expect parturition at about 280 

 days from the last day of the last menstruation. Seeing how- 

 ever that, in many cases, it is uncertain whether the ovum 

 which developes into the embryo left the ovary in connection 

 with the last menstruation or with the first one missed or 

 during the intervening weeks, an exact determination of the 

 duration of pregnancy is difficult if not impossible. 



In some animals the period of gestation is longer, in others 

 shorter than in man, being in the mare about 350 days, in the 

 cow about 280 days, sheep about 150 days, dog about 60 days, 

 rabbit about 30 days. 



Immediately preceding labour a secretion of mucus, coming 

 from the os uteri and at times mixed with blood, is often a 

 sign or ' show ' that the efficient uterine contractions, are about 

 to begin. 



§ 707. The onset of labour is marked by rhythmically 

 repeated contractions of the uterus which most distinctly affect 

 consciousness and are recognized as "labour pains." The first 

 effect of these is the opening up or widening of the os uteri 

 constituting the "first stage of labour." The contractions may 

 perhaps be spoken of as " peristaltic " in character, but the 

 arrangement of the bundles of muscular fibres in the body of 

 the uterus is a complex one, and the gross effect of the contrac- 

 tions is to exert pressure, probably of a fairly uniform kind, on 

 the uterine contents, that is on the amniotic fluid or " waters " 

 enclosed in the " membranes" and surrounding the foetus. These 

 membranes are the amnion, the chorion and the decidua, the first 

 being easily separated from the other two along the loose con- 

 nective tissue joining it to the chorion, and thus forming an 

 inner and outer sheet or membrane. Over the os uteri the 

 decidua consists of decidua reflexa only ; and here the mem- 

 branes with the contained fluid act as a hydraulic plug directing 

 the force of the uterine contractions towards expanding the 

 mouth. As labour goes on a special character of the uterine 

 contractions becomes prominent. In the contractions of which 

 we spoke above as occurring during pregnancy before labour 

 really commences, the relaxation of each muscular fibre follow- 

 ing upon a contraction is a complete one. But in labour the 

 muscular fibres while with each pain they contract and relax, 

 are all the while becoming permanently and progressively 

 thicker and shorter. This change by which the uterine wall 

 becomes progressively thicker and more compact has been 

 spoken of under the not very desirable term " retraction," 

 as distinguished from " contraction," but appears to be a sort 

 of tonic contraction or perhaps rather a residue of contraction 

 like that seen in skeletal muscles under certain conditions ; 



72 



