THE PELVIS, 21 



widest below ; and that in the carnivora it is somewhat cylindrical. This 

 difference in outline is conformable with the shape of the foetal thorax, 

 which in the former is deeper than it is wide, particularly at the period of 

 birth. During parturition, the fcetus of herbivorus animals is so placed, 

 generally, that the withers and shoulders are towards the roof or superior 

 plane of the pelvis ; while the sternum and anterior limbs, which form a 

 larger mass, rest on the floor or inferior plane. The passage of the 

 thorax of the foetus in these animals is, apart from other causes, the 

 chief difficulty in parturition. In the human female, it is the head of the 

 foetus. The thorax of the carnivorous foetus is not nearly so deep, com- 

 paratively ; it is therefore better adapted to pass through the nearly 

 circular canal. 



The term diameter, in obstetrics, is employed to designate the distance 

 between certain points in the pelvic cavity, and by which, practically, we 

 may compare the capacity of that space with the volume of the largest 

 part of the foetus that has to pass through it. 



In the human species, four diameters are usually given for the inlet and 

 outlet of "the pelvis, and some veterinarians also furnish these measure- 

 ments. They are : (i) a vertical or sacro-pubic, from the sacro-lumbar 

 articulation to the ischio-pubic symphysis ; (2) a transverse, passing be- 

 tween the most concave portion of the ilia ; and (3, 4) two oblique, from 

 the ilio-pectineal line of one side to the sacro-articulation of the other. 



Chauveau, in his measurement of the Horse's pelvis, gives these four 

 measurements for the inlet (mean vertical, 834^ inches ; transverse, 8^ 

 inches ; oblique, 8^ inches) ; but for the outlet only the vertical and 

 transverse (mean vertical, 6^ inches j transverse, 7 inches). Rainard, 

 for the inlet, gives three diameters : (i) a siipero-inferior, from the sacro- 

 lumbar articulation to the anterior border of the ischio-pubic symphysis ; 

 (2) a transverse, from the inner surface of the cotyloid angle on one side 

 to the same point on the opposite side ; (3) a vertical, from the middle of 

 the sacrum to the ischio-pubic symphysis in the larger animals, and to the 

 sacro-coccygeal articulation in the smaller. But for the outlet, he has 

 only two diameters : (i) a vertical, from the posterior part of the ischio- 

 pubic symphysis perpendicularly to the sacrum or its prolongation, the 

 coccyx ; (2) a transverse, from one ischial tuberosity to the other. 



The most important diameter is certainly that between the middle of 

 the sacrum and the ischio-pubic symphysis in the larger animals, and the 

 sacro-coccygeal articulation and ischio-pubic symphysis in the smaller 

 creatures. For it must be remembered that the pelvis of the domesticated 

 animals offer a vpry inclined plane, and if, placing it in the position of the 

 human pelvis, we draw a horizontal line from the symphysis towards the 

 spine, it will be found that this line does not touch the sacro-lumbar 

 articulation, but the middle of the sacrum in the large, and the sacro- 

 coccygeal articulation in the smaller animals. This point is the narrowest 

 through which the foetus has to pass, and in which it will meet most re- 

 sistance ; for while the top of its shoulder is towards the sacrum, its chest 

 is resting on the pubis. So that it may be said that this is really the first 

 solid resistance to be overcome in parturition. 



Considering the variations in size in some species, it is not possible to 

 give general measurements for all ; but we may follow the example of 

 Rainard, and give average diameters for different-sized animals. These 

 are tabulated as follows ; the last column, headed " Symphysis," gives the 

 length of the floor of the pelvis. 



