234 THE ARTICULATIONS 



^isc here than elsewhere, iJieLmpvements permitted at this joint are very free, being. ireer. than 



! those between any two lumbar vertebrae. As the diameter of the two contiguous bones is less 

 in the sagittal than in the frontal plane, the forward and backward motions are much freer than 

 chose from side to side. The backward and forward motions take place every time the sitting 

 ;is exchanged for the standing position, and the standing for the sitting posture; in rising, the 

 'back is extended on the sacrum at the sacro-lumbar union; in sitting down it is flexed. 



The articular processes provide for the ghding movement incidental to the extension, 

 flexion, and lateral movements; they also allow some horizontal movement, necessary for the 

 rotation of the vertebral column on the pelvis, or pelvis on the column. The inferior articular 

 processes of the fifth differ considerably from the inferior processes in the rest of the lumbar 

 vertebrae, and in direction they resemble somewhat those of the cervical vertebrae; while the su- 

 perior articular processes of the sacrum differ in a similar degree from the superior processes of 

 the lumbar vertebrae. This difference allows for the freer rotation which occurs at this joint. 

 The sacro-vertebral angle averages 117° in the male, and 130° in the female; while the 

 pelvic inclination averages 155° in the male, and 150° in the female. 



As already stated, the movements at the sacro-vertebral joint are the same as those in other 

 parts of the spinal column, but more extensive, and the muscles which produce the movements 

 \ are those mentioned in the preceding groups which cross the plane of the articulation. 



3. THE ARTICULATIONS OF THE PELVIS 



This group may again be subdivided into — 



(a) The sacro -iliac. 



(6) The sacro-coccygeal. 



(c) The intercoccygeal. 



(d) The symphysis pubis. 



(a) The Sacro-iliac Articulation and Sacro-sciatic Ligaments 



Class. — Diarthrosis. Subdivision. — Arthrodia. 



It is now generally admitted that the sacro-iliac joint is a dia,;r ;^ ]j fnsi.«s. the 

 articular surface of each bone being covered with a layer of cartilage7 whilst the 

 cavity of the joint is a narrow cleft and the capsule is extremely thick posteriorly. 

 The cartilage on the sacrum is much thicker than that on the ilium and the 

 cartilages are sometimes bound together here and there by fibrous strands. 

 The different character of the joint in the two sexes should be noted. Briefly, 

 the female joint has strong ligamentous bonds with but little bony apposition, 

 while the male joint gains its strength by virtue of extensive areas of bony 

 contact and a slighter development of ligaments. This difference is, of course, 

 a physiological one; for some laxity of the joint is demanded during pregnancy 

 and labour. The bones which enter into the joint are the sacrum and ilium, 

 and they are bound together by the following ligaments: — 



Anterior sacro-iliac. Superior sacro-iliac. 



Posterior sacro-iliac. Inferior sacro-iliac. 



Interosseous. 



The anterior sacro-iliac ligament (figs. 271 and 272) consists of well-marked 

 glistening fibres which pass above into the superior, and below into the inferior, 

 ligaments. It extends from the first three bones of the sacrum to the ilium 

 between the brim of the pelvis minor and the great sciatic notch, blending with the 

 periosteum of the sacrum and ilium as it passes away from the united edges of 

 the bones. 



The superior sacro-iliac ligament (figs. 271 and 272) extends across the 

 upper margins of tlu; joint, from tlic ala of the sacrum to the iliac fossa, being 

 well marked along tlu; brim of the pelvis, where it is thickened by some closely 

 packed ril)r('S. lichind, it is far stronger, especially beneath the transverse process 

 of thf fifth lunil)ar vertebra. This ligament is connected with the strong sacro- 

 lumbar ligament, which spreads lateralward and forward over the joint to reach 

 the iliac fossa and terminal line. By some authors it is described as a part of the 

 ilio-lumljar ligament. 



The posterior sacro-iliac ligament is extremely strong and consists essentially 

 of two sets of fibres, dec^p and superficial. The deep fibres (short posterior sacro- 

 iliac ligament) pass downward and medialward from the rough area of the 



