16 



OBSTETPdCAL ANATOMY. 



inferior bis-iliac diameter sometimes slightly exceeds the sacro-pubic, 

 but generally it is less. The oblique, ilio-sacral, or sacro-iliac diameters 

 (Figs. 8, ^, e i, f h), pass from the articulations, the names of which 

 they bear, through the middle of the inlet, to the ilio-pectineal crest on 

 the opposite side — just about the centre of the cotyloid cavity. Their 

 length is between that of the sacro-pubic and superior bis-iliac diameters, 

 and they ai'e only important to note when the inlet approaches a circular 

 outline (Figs. 8, 12), as then the foetus may pass through the cavity with 

 its dorso-sternal diameter corresponding to one of them ; while in more 

 oval pelves it rotates slightly in its progress, its larger axis correspond- 

 ing more or less with that of the inlet— that is, to the dotted lines in 

 Figs. 9, 11. 



PosTEBiOE Opening oe Outlet (Fig. 7, b). — This is also sometimes 

 named the pei-ineal circumference, or recto -^iretliral opening ; it includes 

 in its contour the rectum and vagina, and is related to the vulva and 



Inlet of the Pelvis of the Mare : Narrow Pelvis. 



h, Supero-inferior, or Sacro-pubic Diameter ; c d, Superior Bis-iliac 

 Diameter ; e /, Inferior Bis-iliac piameter ; e i, f h, Oblique, Ilio-sacral, 

 or Sacro-iliac Diameters. 



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 surface of the 

 ischia and posterior border of the sacro-sciatic ligaments. The opening 

 is oval. 



The diameters are ordinai-ily much less than those of the inlet — per- 

 haps to the extent of one-fifth ; but this circumstance has rarely any 

 influence in parturition, as the opening is very dilatable, owing to the 

 relaxation that takes place in the sacro-sciatic ligaments during the later 

 months of pregnancy, and the great mobility of the sacrum and coccyx, 

 which allows the supero-inferior diameter to be increased considerably. 



It may be noted that the pelvis of the female Ass differs but little 

 from that of the Mare, so far as shape is concerned, its inlet being 

 generally oval from above to below, the sacro-pubic diameter slightly 

 exceeding the transverse in measurement. 



