PRESENTATIONS OF THE FCETUS. 235 



Saint-Cyr agrees with Rainard in this interpretation of the real mech- 

 anism of parturition in the Mare. The sternum in being carried back- 

 wards, 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 parturition, the foetus becomes, as it were, elon- 

 gated 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, 

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

 impunity. 



In the larger animals, the pelvis cannot undergo any sensible increase 

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

 through the inlet, which is, in the Mare, an absolutely inextensible bony 

 girdle. Lafosse has sawn through the pubis of Mares about to foal, and 

 he found that during parturition there was only a space of two lines be- 

 tween 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-ischiatic 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 pos- 

 terior 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 animal is well elevated. 



A slight check to expulsion is observed (especially in the Mare) when 

 the croup arrives at the inlet, as this part nearly corresponds in diameter 

 to this opening, being, if any thing, slightly less. However, notwith- 

 standing this, in consequence of the croup being less susceptible of 

 diminution than th'e chest, and although the bones may yield to some 

 extent, friction will occur, more particularly if the croup is largely devel- 

 oped, which it is in some foals. One haunch may pass into the inlet 

 before the other, however, and thus facilitate the passage. 



With the Cow, the mechanism of parturition in this presentation is 

 similar to that in the Mare. Saint-Cyr shows, from actual measurements 

 of Cow and foetus, that the head of the calf can easily pass into the inlet, 

 owing to its less diameter ; and that the principal difficulty is encountered 

 by. the foetal thorax, which is slightly larger in every sense than the inlet.* 



The bicoxo-femoral diameter of the croup slightly exceeds the bis-iliac 

 diameter of the pelvis ; but it is possible that the pelvis of the calf being 

 more cartilaginous and supple than that of the foal, may be submitted to 

 a slight temporary compression. It is to be remarked, however, that the 

 progress of the calf through the pelvis must be more protracted than that 

 of the foal, owing to the greater length of the maternal pubic symphysis, 

 and the more considerable extent of the pelvic walls, as well as the pecu- 

 liar curve in the floor of the pelvis ; though these disadvantages are 



* Saint-Cyr in these observations measured the thorax after the birth of the young creatures, and when 

 the lungs had become expanded. He does not appear to have made any allowance for this expansion, 

 which of course makes a difference in the size of the thorax after birth. 



