PRESENTATIONS OF THE FCETUS. 237 



diameter is sometimes equal, or even superior, to the sacro-pubic diame- 

 ter ; so that it is not always absolutely impossible for delivery to occur 

 spontaneously in these positions ; though it is very true that it is always 

 more difficult, and so?netimes impossible, if the position is not altered. 

 Independently of the disproportion between the diameters of the pelvis 

 and the corresponding diameters of the foetus, here also we find the two 

 salient parts of the latter — the sternum and dorsal spines — jamming 

 against the two resisting parts of the pelvic circumference — the ascending 

 branch of each ilium, and it will be readily seen that in some cases these 

 will prove an insurmountable obstacle. Nevertheless, as a general rule, 

 this obstacle may be easily turned by merely causing the body of the 

 foetus to rotate on its axis, in such a manner that its greatest diameter 

 will be brought opposite the oblique diameter of the inlet, which extends 

 from the ileo-pectineal ridge to the sacro-iliac articulation on the opposite 

 side. Then its entrance into the pelvic cavity, and complete expulsion, 

 is rendered possible. 



4. Mechanism of Parturition in the Posterior Lumbosacral Position. — Of 

 the four positions in which we may have a breech or posterior presenta- 

 tion, only one, in the opinion of Saint-Cyr, is compatible with spontaneous 

 delivery — the lumbosacral ; though even this is denied by other author- 

 ities. The foetus is presented by the breech, the loins towards the sacrum 

 of the mother, the hind limbs in complete extension and entering the 

 inlet, so as to open the passage for the body -, and though this position 

 may appear to be favorable for the expulsion of the young creature, yet 

 it is far less so than the first anterior position. The croup of the foetus 

 is a rounded voluminous mass which does not admit of much compres- 

 sion, and the diameters of which — particularly the transverse — are neatly 

 equal to those of the pelvis ; it is, therefore, not well disposed for passing 

 through the latter, and, in addition, its upper part presses against the 

 sacro-vertebral angle ; while the stifles, which are salient, press against 

 the edge of the pubis, and the hip joints against the branches of the 

 plium. Entrance into the inlet must therefore be slow, difficult, and pain- 

 ful for the mother, and when this first obstacle is overcome and the croup 

 is in the pelvic cavity, the chest has to follow, and to submit to the same 

 compression at the inlet as in the anterior presentation. But this part 

 of the foetus is much less favorably disposed for such a reduction of 

 dimensions in this position ; as the resistance offered by the walls of the 

 maternal pelvis has a tendency to erect the dorsal spines, and to carry 

 the ribs and sternum forward — ail this going to increase the diameter of 

 the foetus in every direction. It is only, then, by direct compression or 

 crushing, that the necessary diminution in the diameters of the young 

 creature can be effected, and not by a kind of physiological decrease, as 

 in the anterior presentation. 



There is also the obstacle offered by the hair of the foetus, the " set " 

 of which is against the direction of movement ; and this obstacle will be 

 greatly increased if the fluids have escaped for some time, and the parts 

 are more or less dry. 



Taking all these considerations into account, it will be seen that in this 

 position, even when birth is possible with extraneous assistance, labor 

 must be long and exhausting, and that the young creature incurs the 

 greatest danger. Labor, however, is more likely to be successful and less 

 tedious if the haunches of the foetus present one after the other at the 

 inlet ; so that a slight obliquity in the presentation makes a great differ- 



