PARTURITION. 



499 



We will follow Saint-Cyr in first studying the mechanism of parturition 

 in this presentation, in which, of the four positions pertaining to it, the 

 vertebrosacral is by far the most frequent and favorable. This we will 

 now notice. 



XVI. Mechanism of Parturition in the Anterior Vertebro-Sacral Position. 

 In this position it has been stated that the foetus presents simultaneously 

 with the head and fore limbs, the back directed to that of the mother and 

 the withers towards the sacrum. AVhen perfectly natural, the head and 

 fore legs first enter the inlet; \ho head is extended, forehead looking up- 

 wards to the sacrum, chin towards the pubis, nose forward, the lower jaw 

 resting on the outstretched limbs, the feet of which extend a little beyond 

 the nose. Then comes the neck, and after it the chest and shoulders, which 

 arrive at the inlet when the nose and feet show themselves at the vulva. 



HiND-LiaiB deviation; anterior presentation. 



In this course it will be observed that, so far as the head and limbs are 

 concerned, there is no difficulty, as the pelvic diameter readily admits 

 them when the soft parts are sufficiently relaxed. With the chest, however, 

 there is difficulty, as its diameter is greater than that of the pelvis ; and the 

 question is, therefore, how it is got through the canal. Saint-Cyr saw four 

 well-bred harness and saddle mares give birth to foals at the Lyons Vet- 

 erinary School. Gestation had been regular, and parturition, which was 

 easy and favorable, did not exceed the ordinary duration. In taking the 

 diameters of the maternal pelvis by the method already described, and 

 those of the fcrtus (dorso-sternal, biscapulo-humeral, and bicoxo-femoral), 

 it was found that in these four instances the biscapulo-humeral diameter — 



