THK l- ELVIS. IS 



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 fcetus enters the pelvic cavity, and a know- 

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

 These dimensions are ascertained by taking the diameter of the openiiif 

 at several points, but two diameters are generally recognised : a siipcro- 

 inferior and a transverse. The supcro-inferior, or sacro-pubic diameter 

 (Figs. 8, 9, a, 6), rather oblique, 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 fcetus when it entei-s the 

 pelvis. It varies with the size of the Mare, but is usually between 

 eight and ten inches. The transverse or bis-iliac 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. 



Fig. 8. 



Ini.kt ok the Pklvis ok thk Mahk : WiDK Tklvis. 



/), Supero-inferior, or S.icro-pubic Diameter ; c d, Superior Bis-iliac 

 Diameter ; e /, Inferior Bin-iliac Diameter ; e i, / /i, Ohlique, Ilio-sacrul, 

 or Sacroiliac Diameters ; J K, Middle Diameter. 



It may be observed that Franck gives two oblique transverse diameters, 

 and Saiut-Cyr, in the new edition of his work, follows him, though the 

 two do not agree in their measurements. I do not see much advantage 

 to be gained from the adoption of these oblique diameters, which render 

 the measurements more complicated, especially in view of the fact 

 that the pelvis varies in form — the transverse diameter not unfrequently 

 exceeding that of the vertical ; but, following Saint-Cyr's example, I 

 give them here, as in some respects they may be useful : 



In this measurement there are two transverse, or bis-iliac diameters, a 

 superior and an inferior. The superior (Figs. H, 9, c, d) is about the 

 upper third of the pelvic cavity, and corresponds to the shoulder and 

 hip joints of the foetus when it is in the f/orso- and lumbo-sacral positions 

 — the most frequent; while the inferior {c, f) is drawn at the lower 

 fourth of the cavity, corresponding to the elbow and stifle joints of tin; 

 foetus. The upper diameter is sometimes less than the sacro-pubic (as 

 in Fig. 9) ; but more frequently it is equal, or even superior to it. Tlie 



