THE PEL VIS. 9 



The ajiterior border is thin and uneven, and is curved like the arc of a 

 circle ; it concurs in forming the anterior circumference of the pelvis. The 

 posterior border is thick and concave, and in front circumscribes the oval^ 

 sub-pubic ox obturator foramen. It is channeled, near the acetabulum, by 

 a fissure that passes obliquely downwards and inwards. The internal 

 border joins that of the opposite pubic bone in the middle line, to form 

 the anterior portion of the ischio-pubic or pelvic symphysis. 



The external ox cotyloid angle is the thickest, and constitutes the largest 

 portion of the roughened depressed surface at the bottom of the ace- 

 tabulum. The internal angle is united to the corresponding angle of the 

 opposite bone. Tho. posterior angle is fused at an early period with the 

 anterointernal angle of the ischium, to form the inner boundary of the 

 obturator foramen. 



The pubis does not alter much in form with age, but retains its convex 

 shape, while the part around the acetabulum is of considerable thickness : 

 a circumstance which tends to diminish the pelvic cavity to a notable 

 depfree. During life, the pubic bones gradually lose their spongy tissue, 

 and to such an extent that in old age it has almost disappeared, and the 

 parts are translucid. 



It may also be well to note that the compact tissue is most abundant in 

 the vicinity of the acetabulum, that cavity being the point where the 

 impulsive efforts communicated to the body by the posterior limbs are 

 concentrated ; at this part, also, ossification commences. 



2. Sacrum. 



The sacrum {os basilare of man) may be said to terminate the vertebral 

 spine posteriorly, and results from the fusion of five vertebrae into a single, 

 voluminous, pyramidal or triangular mass. It encloses the pelvic cavity 

 above, and articulates in front with the last lumbar vertebra, behind with 

 the first coccygeal or tail-bone, and laterally with the ossa innominata. 

 It has an upper 2iX\d a lower face, two lateral borders, 2l base or anterior 

 extremity, sutiimit or posterior extremity, and central cajial, 



ThQ upper face shows the supra-spiyious processes or supra-sacral spine 

 (though the processes only meet at their base). On each side of this spine 

 is a channel in which are four openings — the supra-sacral foramifia, which 

 communicate with others on the inferior face. The lower face is smooth, 

 and slightly concave from before to behind ; this is the roof of the pelvic 

 cavity, and shows traces of its being composed of five bones, as well as 

 offers four foramina for the passage of the sub-sacral nerves. 



The two lateral borders are thick and concave, and posteriorly show a 

 rugged lip. In front is an irregular oblique surface for articulation with 

 the ossa innominata ; this is divided into two portions, the lower of 

 which, slightly uneven and diarthrodial, is the auricular surface ; the 

 upper is for the insertion of the sacro-sciatic ligament. 



The base, or anterior extrefnity, is articulated by a slightly oval and con- 

 vex surface with the last lumbar vertebra, and forms with the spine a 

 salient angle looking down towards the abdominal cavity, named the sacro- 

 vertebral angle. Laterally, it is united with the two coxae, between which 

 it is fixed like a horizonta/ wedge. In front it shows the opening of the 

 spinal canal. 



The summit or posterior extremity likewise offers the opening of the 

 spinal canal, and a surface for articulation with the first tail-bone. The 

 central ca7ial\s a continuation of that in the other vertebrae, for the passage 



