THE PELVIS. 1 7 



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 previously- 

 emptied bladder may 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 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 cause the cramps pregnant animals 

 experience towards the termination of gestation. The lateral planes are 

 readily dilatable during parturition. 



Anterior Opening or Inlet. — This, which may also be designated the 

 bri77i^ afiterior circumference, ftr abdominal opening of the pelvis, is nearly 

 circular, or slightly 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 articulations 

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

 pubic bones ; and on each side,-by the ileo-pectineal crest and a portion 

 of the inner aspect of the ilia. Owing to this circumference being entirely 

 bony, and to the solidity of the articulations between the different bones, 

 the inlet of the pelvis cannot be dilated to any appreciable degree, even 

 under the most violent efforts, and supposing the sacro-iliac and ischio- 

 pubic ligaments to become softened and relaxed before pregnancy — a 

 change which must be indeed rare in the Mare. 



It is by the inlet that the foetus enters the pelvic cavity, and a knowl- 

 edge of its dimensions is therefore of much moment to the obstetrist. 

 These dimensions are ascertained by taking the diameter of the opening 

 at several points, but two diameters are generally recognized : a supero- 

 inferior and a trafisverse. The sicpero-inferior, or sacro-pnbic diameter, is the 

 width between the sacro-vertebral angle and the symphysis pubis. This 

 is generally the largest diameter, though exceptions are met with now and 

 again ; it is the diameter which should receive the widest part of the foetus 

 when it enters the pelvis. It varies with the size of the Mare, but is 

 usually between eight and ten inches. The transverse diameter is measured 

 from one ileo-pectineal crest to another, and is generally less than the 

 supero-inferior, though sometimes it may be equal, or even greater. It is 

 from seven to nine inches. 



Posterior Opening or Outlet. — This is also sometimes named the 

 perineal circumfereftce, or recto-urethral openifig; it includes in its contour 

 the rectum and vagina, and is related to the vulva and anus, which are 

 external to it. Owing to the horizontal direction of the Mare's pelvis, 

 this outlet is limited above by the apex of the sacrum and the base of the 

 coccyx ; below, by the ischial arch formed by the junction of the two 

 ischia ; and, laterally, by the upper face of the ischia and posterior border 

 of the sacro-sciatic ligaments. The opening is oval. 



The diameters are ordinarily much less than those of the inlet — perhaps 



