XIII ] 



FEMALE GENERATIVE ORGANS 



189 



mechanical stimuli. During the anocstrum tjie \itcrine contrac- 

 tions are ahvaj-s very slight and infrequent, and it is only during 

 pregnancy and with the approach of parturition that they become 

 considerable. At such times, as already- mentioned, the muscles 

 of the uterus are very greatly developed. 



The dilatation of the os marks the first stage of labour. In the 

 second stage all the uterine muscles contract, as well as those 

 of the abdominal wall and the diaphragm. These coordinated 

 muscular movements expel the foetus from the uterus to the 

 vagina, and thence to the exterior, the final pro})ulsion from the 



Fig. 99. Third position preparatory to parturition (after Franck, from Fleming, 

 Messrs Bailliere, Tindall and Cox). 



vagina being due chieEy to the contractions of the abdominal 

 muscles. The foetal membranes which contain fluids and serve 

 as a sort of elastic bag round the yoimg animal, thus protecting 

 it from mechanical shock or jar, are the first to appear. The 

 'water bag' then ruptures, some of the fluid escaping, and the 

 young animal soon afterwards makes its apiDcarance. 



The third stage of parturition consists of the expulsion of the 

 foetal membranes Avhich together constitute the afterbirth. This 

 is done by further contractions of the uterine muscles accompanied 

 by the action of the diaphragm and muscles of the abdomen. 

 Sometimes, as not infrequently haiipens in the mare, the 3'oung 



