THE I'KIA'IS. 6 



whole length by a wide groove which reaches the bottom of the acetabu- 

 lum, and lodges the pubio-femoral ligament and a very large vein. 



The anterior border is thin and uneven, and is curvetl 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, or obturator foramen. It is channelled, near the aceta- 

 bulum, 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 or cotyloid angle is the thickest, and constitutes the 

 largest portion of the roughened depi'essed surface at the bottom of 

 the acetabulum. The internal angle is united to the corresponding 

 angle of the opposite bone. The posterior angle is fused at an early 

 period with the antero-internal 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 degree. 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 connncnces. 



2. Sacru7H. 



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

 spine posteriorly, and results from the fusion of five vertebra) 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 2ipper and a lojccr face, tiro lateral 

 borders, a base or anterior extremit;/, summit or posterior cxtremiti/, and 

 central canal. 



The upper face shows the supra-spinous 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 foraminci, 

 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 extremit;/, is articulated by a shghtly oval and 

 convex 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 to the two coxa, between 

 which it is fixed like a horizontal wedge. In front it shows the open- 

 ing of the spinal canal. 



