Vertebral Column 41 



structure can be traced down through it until it reaches its end in the badly developed 

 elements of the fifth segment. In addition to these fundamental parts of the lateral 

 mass, there are epiphyses that cover its lateral aspect (Fig. 33). There are also 

 ossifications of inter-transverse, costo-transverse, and other ligaments. 



The articular surface for the sacrum on the innominate bone extends back on to 

 the posterior inferior spine of that bone, so that this bony point is opposite the third 

 sacral segment : the great sciatic ligament is attached above to the inferior spine, and 

 from this extends on to the lower part of the transverse tubercle of the third segment. 

 The upper part of the tubercle has the superficial oblique fascicles of the sacro-iliac 

 ligaments attached to it. Below this level the great sacro-sciatic ligament is fastened 

 to the posterior aspect of the lateral margin of the bone and to the transverse tubercles, 

 extending down to the coccyx, and more superficial fibres extend further in to obtain 

 attachment to the articular masses and cover some outer fibres of the Multifidus 

 spinae arising from the bone. Gluteus maximus spreads from the ilium down this 

 ligament and thus gets indirect origin from the bony points of the lower part of the 

 sacrum (see Fig. 34). 



Fibres of Multifidus spinae can be found arising in the sacral groove as far down as 

 the fourth segment, and extending outwards between the third and fourth foramina : 

 above this the fibres reach the sacro-iliac ligaments externally, so that the more super- 

 ficial Erector spinae is separated by it from the sacrum and only reaches bone (ilium) 

 outside the ligamentous area, though it has some slight attachment through its covering 

 aponeurosis to the median spinous ridge. 



It thus comes about that the posterior primary divisions of nerves, coming through the first 

 three foramina, are in the substance of Multifidus spinae and are connected by loops here, on the 

 bone : their outer branches run down and out to get on to the great sciatic ligament deep to Gluteus 

 maximus. Lower down the loops between the third, fourth and fifth posterior divisions lie on the 

 bone. 



At the extreme edge below the articular surface is the attachment of the Coccygeus 

 fibres, extending down to the coccyx (see Fig. 35). The back and upper fibres of 

 this sheet are ligamentous and constitute the small sacro-sciatic ligament, so that 

 this ligament separates the muscle from the great sciatic ligament, being in contact 

 to some extent and fused with the latter and being continuous with the former. 



The fifth sacral nerve emerges at the side of the lower sacral aperture, and its anterior primary 

 division turns outwards and forwards, below the cornu and behind the last segment of the bone. 

 At the outer border of this it reaches the plane of the Coccygeus and pierces this to appear on its 

 pelvic aspect, where it is joined by the descending filament from the fourth sacral and turns down 

 close to the coccygeal attachment of the muscle. The anterior division of the coccygeal nerve runs 

 a similar course at a lower level, behind the first coccygeal body, piercing the muscle lower down to 

 join the fifth sacral nerve in front of it : the nerve thus composed pierces the Coccygeus again near 

 the tip of the coccyx to supply the skin in that region. 



The front surface of the bodies particularly in the middle segments may show 

 longitudinal markings, which indicate that some at least of the fibres of the anterior 

 common ligament are involved in the general ossification of cartilage and ligament 

 that consolidates the bone ; but in any case the greater part of this ligament is con- 

 tinued down on the front of the bone, forming the anterior sacro-coccygeal ligament at 

 its lower end : the middle sacral artery runs down on the ligament. 



At the side of the bodies the parietal pelvic fascia is attached to the margin of 

 the foramina, so that the issuing nerves lie deep to the fascia : they lie on the Pyri- 



