During labour, the fcetal head enters the brim of the pelvis in the oblique diameter, which being less encroached on by the muscles is the best adapted to receive it, as well as by its correspondence in form to the brim in that direction. Then traversing the pelvic axis, it passes, first downwards and backwards, and then, being turned forwards by the sacro coccygeal curve, escapes under the pubic arch and through the inferior outlet. The fcetal occiput is directed generally in the left oblique diameter, towards the left ilio-pubic junction (in 69 per cent. of the cases —Naegele — to SO per cent. — Boivin), ancl in its progress is twisted gradually forwards and towards the median line by the impingement of the parietal protuberance upon the inclined plane of the pyriformis muscle and upon the projecting ischial spine, until it emerges under the sym physis pubis, around which it turns vertically as round a centre ; the real centre of its motion, however, being, as we have before scen, a little in front of and below the pubis. The more anterior part of the fcetal head, then, traversing the circumference of the sub pubic circle, extends the coccyx, and passes between the tuber ischii, distending the sacro sciatic ligaments and perineum, and turning, as it does so, on its own transverse axis.
In a well-formed woman, according to Naegele, the superior plane of the pelvis will be horizontal when the trunk is between the sitting and recumbent positions (i. e. when it forms an angle of 30° with the horizon). In such a position traction on the head of the child should be perpendicular.
In sustaining these evolutions, the pelvic circle necessarily is exposed to a force not hitherto considered, viz. pressure from within. In well-formed pelves this pressure will be exerted equally on all parts of the circum ference, frotn the adaptation, before mentioned, of the child's head to tbe form of the pelvis. The strain, however, vvill be most evidently exerted upon the ligaments of the pubic and sacro-iliac symphyses.
The question whether these ligaments yielded during labour sufficiently to enlarge materi ally the diameters of the pelvis, is one which has attracted the attention of anatomists and obstetricians very much, particularly about the latter end of last century, sv ben Sigault prcposed, in lieu of the Cmsarian section, the section of the synaphysis pubis, with a view of affording greater pelvic diameters.
Arnong tbe older writers on this subject Mauriceau, Peu, Lamotte, Vesalius, Varan dens, Menard, and Voigt denied that separa tion of the pubic bones occurred during labour.
Some believed it to occur only in young primiparous females ; others in prirniparous females of advanced age ; and others again only in pelvic distortions or peculiar circum stances of pregnancy.
Ambrose Pare, Pineus, Bauhineus Riveanus, Diemerbroek, Arniseus, Bianchi, Gregoire Pineau, Duvernay, Benin, Levret, Santorini, Spigelius, and Smellie, have observed this se paration in the dead parturient woman ; and Guillemeau, Hildanus, Van Solingen, Ves lingius, Puzos, Soumain, Bikker, Arnauld, and Morgagni in the living subject. Pare and Peu asserted that they had seen cases where the ilia had been separated from the sacrum; and Smellie relates a case where great pain at the sacro.iliac joints rendered this probable.
Of those who admit that relaxation of the ligaments and consequent separation do take place, Boivin, Louis, Severin, Pineau, and Meckel consider it to depend upon softening, thickening, and loosening of a single inter pubic fibro-cartilage, and deposition of fluid in its meshes ; Meckel with Antoine Petit, deny ing the frequent existence of a separation be tween two plates of cartilage. ButTenon found, that although, in most cases, the inter-pubic fibro-cartilage was single in the male, in woman it was generally double, and contained a slit or cavity, with no connecting fibrous tissue between the middle of the plates. And he found this to be the case in the young as well as the old, and before pregnancy as well as after ; sometimes the slit was capillary, but in one female, recently delivered, the cavity would admit the forefinger. In none of his
examinations did he find any thickening, soften ing, or laxity of the fibro-cartilage itself, how ever recent the accouchment, although the external investing ligaments of the joint were relaxed and elongated. He considers, that if in females where but one fihro-cartilaginous plate is present, separation of the bones occurs during labour, it must be by rupture of the fibres of the disc or its separation from the bone. It is, however, difficult to comprehend why elongation of these fibres should not take place as well as of those of the investing liga ments.
Tenon also asserts, that he has observed relaxation of the pubic ligaments, even in the male, during life. Sandifort considered that in women who have borne many children the pelvic ligaments become permanently looser. In a female aged 79, dissected by Cruveilhier, the symphysis pubis was exceedingly moveable, the sub-pubic ligament had disappeared, and a fibrous capsule invested the joint. Scemmer ring believes that separation of the pubes is not rare, even during easy labours, and that they remain permanently loose, and the pelvic diameters larger, after many labours. Ac cordinn. to the last author, the sacro-iliac joints have also been found separated by a cavity of the width of an inch in females who have died during parturition. But it is a question, whether this was not caused by disease and deposit of pus. In a woman mentioned by Frank, in Textors Neuetn. Chiron. (vol. i. p. 261.), the pelvis was so moveable every tirne she became pregnant, that she could not stand upright. Instances of separation of the pubic joint during labour are also mentioned by Eichelburg in Rust's Magazine (vol. xvii. part iii. p. 550.), and by Nicholson in Trans. of Physic. in Ireland (vol. iv. 1824). Dr. William Hunter, in his memoir upon this subject, gives two cases of parturient women, in which the fibrous tissue connecting the cartilages was replaced by a ca vity lined by a perfect synovial membrane, and forrning a perfect arthrodial joint; and he concludes, that a certain relaxation of the pelvic ligaments takes place in the latter rnonths of pregnancy and during labour, al lowing of a slight increase of the pelvic ca pacity, and a yielding to the shape of the fcetal skull.
On examining the pelves of five women who had died soon after delivery, Dr. Knox of Edinburgh found, in all, a remarkable relaxa tion of all the ligaments of the pelvic joints ; in one the bones could be made to slide over each other. This anatomist also inclines to the opinion that this process is a regular and healthy one in the parturient female. This opinion is also supported by the post-mortem researches of the celebrated Rokitansky of Vienna, who considers that, in parturient women the pelvic ligaments become soft, relaxed', and stretched.
In addition to the testimony on this side of the question, M. Senoir read an essay on the " Articulations of the Female Pelvis,' at the Academy of Medicine of Paris, on the 1st of April, 1851 ; in which he concludes, 1st. That the articulations of the pelvis proper should not be considered as amphiarthroses, but as arthroses ; and, 2ndly. l'hat analogy of structure and composition leads him to think that an effusion of synovial fluid from the membrane lining the joints, causes the sepa ration of the bones sometimes observed. (Bulktin de Academie Nation. de Modecine, t. xvi. No. 13. April 15th, 1851.) It is, however, considered by Baudelocque, Boyer, Burns, Dewees, Denman, and I think most English writers on Obstetrics of the present day, that, in common cases, no sensible relaxation of the pelvic ligaments takes place, and that, when such relaxation does occur, it should be rather considered in the light of a morbid condition, as it adds very little to the diameters, and is attended with severe in conveniences from rupture of the sacro-iliac ligaments.
Notwithstanding, comparative anatomy, par ticularly in Cows, Seals and Guinea-pigs, to be presently considered, presents us with a strong analogical proof in favour of the doc trine of some parturient relaxation of the pelvic ligaments in women.