SACRAL VERTEBRA. 15 



The ariknlar processes are thick and strong. Their articular surfaces 

 are vertical ; those of the superior joair concave, look backwards and 3 — \ 



inwards, those of the inferior, convex, look forwards and outwards. 

 The superior i^air are fdrtlTef "apart than the inferior, and embrace the 

 inferior pair of the vertebra above them. From each superior articular 

 process, a tubercle projects backwards, Avhich corresponds to the 

 internal tubercles of the dorsal transverse processes, and is called the 

 mammiUarii process (metapophysis, Owen). 



The fonnwn is large and triangular, or widely lozenge-shaped. 



Sacral Vertebrs. — These by their union in the adult, form the 

 OS sarrum, but in youth they present the elements of five distinct 

 vertebrse. The sacrum is placed below the last lumbar vertebra, and 

 articulates laterally with the two ossa innominata, thus completing 

 together with the coccyx the wall of the pelvis above and behind. The 

 uppermost vertebra is the largest, those which follow become rapidly 

 smaller and the fifth is rudimentary. Hence the sacrum has the form 

 of a triangle with its base directed upwards. It is concave and smooth 

 in front, convex and uneven behind. The direction of its surfaces is 

 very oblique, its ventral aspect looking somewhat downwards, and 

 forming above at the place where it joins the last lumbar vertebra, the 

 projection termed j^romonfor//. The dorsal or posterior surface looks 

 slightly upwards. 



Fig. 11. 



Fig. 11. — Sacrum of the Male, viewed prom 



BEFORE. I 



1, 1, four transverse ridges, indicating tlie j^lace of 

 original separation of the bodies of the five sacral 

 vertebra ; 2, anterior sacral foramina ; 3, i, lateral 

 surface ; 5, a notch which, with the coccyx, forms a 

 passage for the fifth sacral nerve ; 6, oval surface of 

 the upper part of the sacrum for articulation with the 

 botly of the last luml^ar vertebra ; 7, superior articular 

 processes ; 8, inferior oval surface for articulation 

 with the coccyx ; + , inferior lateral angle. 



The ventral surface is concave from above downwards, and slightly 

 so from side to side. It is traversed horizontally by four ridges, A\hich 

 indicate the places of union of the bodies of the five vertebra, and at 

 the extremities of which are situated on each side four foramina called 

 anterior sacral These foramina widen externally into grooves, and 

 diminish in size from above downwards. 



The dorsal surface is convex, very uneven and somewhat narrower 

 than the ventral. It presents along the median line four small 

 eminences, the spinous processes, usually more or less connected, so as 

 to form a ridge. Below the last spinous process is a triangular opening, 

 the termination of the spinal canal, the lateral margins of which are 

 formed by the imperfect lamina3 of the fifth sacral vertebra, and present 

 a pair of tubercles, the sacral cornua, which project downwards, and 

 articulate with the cornua or horns of the coccyx. On each side of the 

 ridge of spines the surface is smooth and hollowed in the position of 

 the united laminre, and beyond this surface are two rows of eminences, 

 the inner of which corresponds to the articular and mammillary pro- 

 cesses of the lumbar vertebrtc, the outer to the transverse processes. 



T ^^ 



