14 



OBSTETEIGAL ANA TOMY. 



subsacral vessels, and sympathetic nerves. It is slightly concave 

 longitudinally. 



The inferior plane, or floor of the pelvis, is constituted by the upper 

 surface of the pubic bones and ischia. It is rectilinear from before to 

 behind, and concave from side to side. The symphysis pubis occupies 

 the median line; it is salient, and varies in length according to the 

 size of the animal, being usually about six or seven inches. In front, 

 at the pubis, is a depression more or less marked, in which the pre- 

 viously-emptied bladder can be lodged during the passage of the foetus. 

 On each side is the obturator foramen, which is partly closed by the 

 internal obturator muscles, and through which the obturator vessels 

 and nerves make their exit. 



The two lateral planes are formed by the inner surface and spine of 

 the ischia, and in great part by the sacro-sciatic ligaments ; the sciatic 

 notches belong to them, and they are traversed from before to behind 



rig. 7. 



Diagram of the Mare's Pelvic Axis. 

 A, Inlet ; B, Outlet, 



by the obturator vessels and nerves, and pierced by the gluteal and 

 ischio-muscular vessels and nerves, the internal pudic and the great 

 and small sciatic nerves. As has been stated, it is the compression of 

 these nerves by the uterus and its contents which causes the cramps 

 pregnant animals experience towards the termination of gestation. 

 The lateral planes are readily dilatable during parturition. 



Anterior Opening or Inlet (Figs. 7, 8, 9). — This opening, which is 

 continuous with the abdominal cavity, and may also be designated the 

 brim, anterior circumfereyice, or abdominal ojjening of the pelvis, is nearly 

 circular, or shghtly oval, the widest part corresponding to the symphysis 

 pubis. It is a little obliquely inclined downwards and backwards, and 

 is limited above by the anterior border of the sacrum and its articula- 

 tions with the lumbar vertebra and ilia ; below, by the anterior border 

 of the pubic bones ; and on each side, by the ileo-pectineal ci-est and a 

 portion of the inner aspect of the ilia. Owing to this circumference 

 being entirely bony, and also to the solidity of the articulations between 

 the different bones, the inlet of the pelvis cannot be dilated to any 

 appreciable degree under the most violent efforts, even supposing the 



