678 MAMMALIAN ANATOMY 



ventral wall lies beneath the movable posterior part of the dorsal wall. 

 In this way the least possible resistance is offered to the passage of the 

 young through the pelvic cavity at birth. The pelvis is almost twice 

 as long from the tuberosity of the ischium to the crest of the ilium as 

 it is high from the crest of the pubes to one of the posterior articular 

 processes of the sacrum. Its maximum width, at the tuberosities of 

 the ischia, is slightly greater than its height. It is usual to divide the 

 pelvis into two parts, the false pelvis and the true pelvis. The line of 

 division between the two parts is seen on the inner wall only. 



The false pelvis is almost absent in the cat, and consists of a dorsal 

 half only. Its lateral walls are formed on each side by the part of 

 the ilium anterior to the ilio-pectineal line ; its dorsal wall is the 

 narrow strip of the sacrum anterior to an imaginary continuation of 

 the pectineal line upward and inward across the ventral surface of 

 the lateral mass to the lower outer corner of the anterior articular end 

 of the body. 



The true pelvis comprises the greater part situated posterior to this 

 line. It presents a cavity bounded by bony and muscular walls and 

 having an anterior opening or inlet and a posterior opening or outlet. 



The inlet of the pelvis is at the brim of the pelvis ; it is formed on 

 each side, from below upward, of the border of the pubes, the ilio- 

 pectineal line, and the line of the greatest antero-posterior convexity 

 on the sacral lateral mass. It is a regular oval opening, and its antero- 

 posterior diameter, from the pelvic symphysis to the sacrum, is almost 

 a fourth greater than the greatest transverse diameter, which is found 

 between points just above the pectineal eminences. The oblique 

 diameter is measured from the ilio-pectineal eminence of one side to 

 the sacro-iliac articulation on the other side. The plane of the inlet 

 of the pelvis faces almost directly forward and only slightly down- 

 ward, and is very oblique to the long axis of the general cavity of the 

 pelvis, which runs downward and backward. 



The pelvic cavity is almost circular in front and is triangular 

 behind ; its dorsal wall is twice as long as the ventral wall. It is 

 bounded above by the ventral surfaces of the sacrum and of the four 

 caudal vertebrae ; at each side, in front, by the inner surface of the 

 ilium, and behind by the inner surface of the body of the ischium. 

 Its lower wall is formed by the pubes and the ascending ramus of 

 the ischium. Its lateral walls are parallel, and the upper wall is, 



