200 THK jnTICl'LATlOXS 



X<'ar the spine the surfaces look directly l)ackwar(ls and forwards, hut at the 

 ilium, the ligament gets somewhat twisted, so that the jjosterior surface looks a 

 little upwards, and the anterior looks a little downwards. 



The anterior surface forms part of the posterior houndary of the false pelvis, 

 and overlies the u]iper part of the ]iosterior sacro-iliac ligament; the posterior 

 surface forms i)artof the floor of the sjnnal groove, and gives origin to the multijidu.-i 

 ^pitiiv nuiscle. Of the l)orders, the upper is ohlique, has the anterior lamella of the 

 lumhar fascia attached to it, and gives origin to the qvadmtus himhorvm ; the lower 

 is horizontal, and is adjacent to the upper edge of the sncro-lumhar ligament; while 

 the inner is crescentic, and forms the outer houndary of a foramen through which 

 the fourth liimlxrr nerve passes. 



The arterial supply is very free, and comes from the last luml^ar, ili()-luml)ar, 

 and lateral sacral. 



The nerve-supply is from the sympathetic, as well as frf)m twigs from the 

 fourth and lifth lumhar nerves. 



Movements. — The angle formed l)y the sacrum with the spinal column is called 

 the sacro-vertel)ral angle. The pelvic inclination does not depend entirely upon 

 this angle, hut in great part upon the obliquity of the innominate bones to the 

 sacrum, so that in males in whom the average pelvic obliquity is a little greater, 

 the average sacro-vertebral angle is considerably less than in females. 



The sacro-vertebral angle in the male shows that there is a greater and more 

 sudden change in direction at the sacro-vertebral union than in the female. A part 

 of this change in direction is due to the greater thickness in the anterior part of the 

 intervertebral substance between the last lumbar vertebra and the sacrum. Owing 

 to the greater thickness of the intervertebral disc here than elsewhere, the move- 

 ments permitted at this joint are very free, being freer than those between any two 

 lumbar vertebne. As the diameter of the two contiguous bones is less antero- 

 l)Osteriorly than laterally, the forward and backward motions are much freer than 

 the lateral ones. The l)ackward and forward motions take place every time the 

 sitting is exchanged for the standing position, and the standing for the sitting pos- 

 ture; 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 gliding movement incidental to the 

 extension, flexion, and lateral movements; they also allow some horizontal move- 

 ment, necessary for the rotation of the spine on the pelvis, or ])elvis on the spine. 

 The interior articular })rocesses of the fifth differ consideralily from the inferior 

 jtrocesses in the rest of the lumbar vertebric, and in direction they resemble some- 

 what those of the cervical vertebra?; while the superior articular processes of the 

 sacrum differ in a similar degree from the superior processes of the lumliar vertebnw 

 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. 



3. THE ARTICULATIONS OF THE PELVIS 



This group may again be subdivided into — 



('0 The sacro-iliac. 

 (f>) Tlie sacro-coccygeal. 

 ('•) The intercoccygeal. 

 {<}) The symphysis pubis. 



(<i) The S.vciio-iLiAc Synchondrosis and Sacro-sciatic Lig.vments 



Class. — Amphiarthrosis. 



Like the symphysis ])ubis, the sacro-iliac sj-nchondrosis is an amphiarthrodial 

 joint, but it differs from it in having an interosseous ligament as well as an 



