390 I'AKTUIUTION. [BOOK iv. 



expect parturition at about 280 days from the last day of the last 

 ni.'iistruation. Seeing however that, in many cases, it is un- 

 ivrtain whether the ovum which developes into the embryo left 

 the ovary in ronnt-rtion with the last menstruation or with the 

 tir-t one missed or during the intervening weeks, an exact deter- 

 mination 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 liSO 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 

 simi or ' show ' that the efficient uterine contractions are about 







to begin. 



$ 966. The onset of labour is marked by rhythmically re- 

 peated 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 con- 

 stituting the " first stage of labour." The contractions may per- 

 haps be spoken of as " peristaltic" in character, but the arrange- 

 ment of the bundles of muscular fibres in the body of the uterus 

 is a complex one, and the gross effect of the contractions 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 fcetus. These membranes are 

 the aninion, the chorion and the decidua, the first being easily 

 separated from the other two along the loose connective 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 membranes 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 following 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 ; at each recur- 

 ring " pain " the shortening of the muscular fibres starts so to speak 

 from this more permanent shortening instead of from complete 

 relaxation, and the return is to it not to complete relaxation. 



This more permanent tonic contraction or "retraction" does 



