1 8 OBSTETRICAL ANATOMY. 



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 may allow the supero- 

 inferior diameter to be increased considerably. 



The Cavity of the Pelvis. 



The cavity of the pelvis is the space between the inlet and outlet. In 

 the human female, it lodges nearly the whole of the uterus, and in the early 

 days of pregnancy the foetus also. This is not the case with the domesti- 

 cated animals, owing to their different attitude, until the act of parturition 

 carries the progeny there.* With its two openings, the pelvic cavity is 

 capable of more or less increase in capacity in every direction, through 

 relaxation of the pubic and sacro-iliac articulations and sacro-sciatic liga- 

 ments. The yielding of the latter is very noticeable in the larger animals 

 immediately before parturition, as well as the elevation of the coccyx by 

 the body of the foetus in its passage outwards. It is also a fact of daily 

 observation that the pelvis permanently widqfis in animals which have had 

 young frequently ; this accounts for the peculiar rocking gait they exhibit 

 in progression, which, in some of the domesticated creatures at least, is 

 no doubt due to persistent relaxation in the articulations above men- 

 tioned. 



It may be again observed that the anterior margin of the flopr of this 

 cavity is nearly straight, and its posterior border is deeply cut into by the 

 ischial arch, while the floor itself often offers some diversities. For in- 

 stance, it may be convex in front and concave behind, or vice versa, the 

 concavity being separated from the convexity by a transverse ridge, which 

 may also be represented by a series of small conical eminences ; or the 

 floor may be a smooth plane sloping upwards from before to behind, 

 with a kind of raised border surrounding the anterior contour of the ob- 

 turator foramen. 



Differences in other Animals. 



Cow, 



In the Cow, the pelvis is longer than in the Mare, and less vertical ; the 

 ischio-pubic symphysis is also longer, and instead of being straight is very 

 curved ; so that the floor of the pelvis is concave in every direction. The 

 ischial arch is more deeply cut at the symphysis, and the posterior borders 

 of the ischia join at an acute angle or V-shape, the opening being supero- 

 posterior. The external border of these bones is higher, and the sciatic 

 spine or supra-cotyloid crest is thinner and more elevated. So that, alto- 

 gether, the bony parietes of the Cow's pelvis are more extensive, com- 

 paratively speaking, than the Mare's. The sacral surface is more concave, 

 and the sacro-sciatic ligaments wider, though shorter. 



The pelvic cavity of the Cow is also less wide, when compared with its 

 height. The diameters of the inlet — which is more oblique than in the 

 Mare — are very unequal \ the difference between the sacro-pubic and the 

 transverse, according to Saint-Cyr, being one-third (nine and six incli^s). 

 The dimensions of the outlet are more equal, and are about those of the 



* Girard thought that, in the Bitch, one of the young in the body of the uterus might occupy this space ; 

 but Rainard could not verify this, all his examinations of pregnant animals which had died before bring- 

 ing forth their young proved the body of the uterus to be quite empty. 



