252 NORMAL P ART U HIT ION. 



ing at the pelvic entrance, come in contact with the ascending branches 

 of the ihum, and are thrown back somewhat, leaving the front part of 

 the chest free, and thus diminishing its diameter. He also adds that 

 the witliers first enter beneath the sacrum ; that the sternum below is 

 pushed back by the anterior border of the pubis, and the chest in this 

 way submits to a process of elongation which notably diminishes its 

 vertical diameter. 



Saint-Cyr agrees with Eainard in this interpretation of the real 

 mechanism of parturition in the Mare. The sternum, in being carried 

 backwards, also pulls back the ribs attached to it, and this not only 

 diminishes the chest in a vertical, but also in a horizontal direction, 

 as is witnessed in studying the mechanism of respiration in the 

 living animal, in which, during expiration, the chest decreases in 

 width and depth. When the chest is so altered during parturi- 

 tion, the foetus becomes, as it were, elongated by this part being 

 depressed ; an alteration which occurs all the more readily, from 

 the bones composing the thorax being soft and supple, and the 

 organs they enclose (the lungs) not being so developed as they are 

 immediately afterwards ; so that a moderate amount of pressure, pro- 

 vided it is not too long continued, may be borne with comparative 

 impunity. 



In the larger animals, the pelvis cannot undergo any sensible in- 

 crease in size during the passage of the deepest portion of the foetal 

 body through the inlet, which is, in the Mare, an absolute inextensible 

 bony girdle. Lafosse has sawn through the pubes of Mares about to 

 foal, and he found that during parturition there was only a space of 

 two lines between the sawn margins. So that it is the body of the 

 foetus which has to accommodate itself to this part of the passage at 

 this stage of delivery. 



When, however, it has passed through the inlet, extensibility of the 

 maternal tissues can, and does, take place, and permits an enlargement 

 of the canal. The wide sacro-sciatic ligaments which enclose the pelvis 

 laterally, are softened and more elastic during birth ; the sacro-iliac 

 and sacro-lumbar articulations are increased in mobility ; and even the 

 posterior part of the ischio-pubic symphysis may become slightly 

 relaxed. So that when once approaching the outlet the progress of 

 birth is more rapid, and this progress may be aided if, as is pointed 

 out by Lafosse, the tail of the mother is well elevated. 



A slight check to expulsion is observed when the croup arrives at the 

 inlet, as this part nearly corresponds in diameter to this opening, being, 

 if anything, slightly less. However, notwithstanding this, in conse- 

 quence of the croup being less susceptible of diminution than the chest, 

 and although the bones may yield to some extent, friction will occur, 

 more particularly if the croup is largely developed, which is the case in 

 some Foals. One haunch may pass into the inlet before the other, how- 

 ever, and thus facilitate the passage. 



2. Mechanism of Parturition in the Dorso-ilio-sacral Positi())}s. — 

 These positions are two in number and symmetrical, and probably are 

 frequent at the commencement of birth in the anterior presentation, 

 when the width of the pelvis exceeds its depth — the oblique diameters 

 being then greater than the vertical — so that the foetal thorax enters 

 even more easily, and it is only when the croup reaches the inlet that 

 the foetus is compelled to rotate slightly on itself to assume the first 

 position on its passage outwards ; when the depth of the pelvis is- 



