8 Veterinary Obstetrics 



ilio-^acral joint. It plays a very important part in the large herb- 

 ivora, especially in the mare, where it sometimes ruptures during 

 advanced pregnancy, resulting in a hernia of the gravid uterus 

 and destroying the normal relation of the pelvis to the spinal 

 column. (See Rupture of Prepubian Tendon, and Figs. 82, 83, 

 84 and 85.) In addition to these ligaments, the great dorsal and 

 psoas muscles serve an important function in maintaining the re- 

 lations between the pelvis and spine. 



The pelvic cavity is somewhat conical in form in its antero- 

 posterior diameter, with the base of the cone presenting forwards 

 so that the pelvic inlet is somewhat larger than its outlet, a dif- 

 ference which is counter-balanced fully by the fact that the inlet 

 is really the only non-extensible portion of the pelvis so that, in 

 practice, other things being normal, if a fetus can enter this por- 

 tion it can pass through the outlet by causing its dilation. This 

 distensibility is dependent upon the yielding character of the 

 sacro-sciatic ligament, as mentioned above. In woman, the rela- 

 tions between the sacro-pubic and bisiliac diameters of the pelvis 

 are inconstant, resulting in a tortuous passage, which leads to a 

 rotation of the fetus on its long axis during parturition in order 

 to keep the greater dimensions of the fetal body in harmony 

 with those of the bony girdle through which it is passing, while, 

 in our domestic animals, the cavity is rectilinear and the fetus is 

 expelled in a direct, instead of a spiral line. 



In cross section, the pelvic cavity is oval in outline, departing 

 but little from circular, its perpendicular usually being slightly 

 greater than the transverse diameter, though these relations may 

 be reversed. The relations between the perpendicular and 

 transverse diameters do not vary materially between the inlet and 

 outlet, the former being usually the greater for the entire length 

 of the passage. The pelvic channel is direct and almost horizontal 

 but usually directed more or less obliquely upwards and back- 

 wards from the inlet to the outlet. This is especially nota- 

 ble in the cow, where the pelvic floor is quite oblique and concave 

 from before to behind and is further emphasized by the promi- 

 nent ischial tuberosities, placed so closely together that, for prac- 

 tical obstetric purposes, the summits of these may, in some cases, 

 virtually mark the floor of the pelvic outlet because they are too 

 close to each other to permit of the ready passage of the fetus 

 between them. In the mare the pelvic floor is almost level and 



