PELVIS. 



143 



the line a, b ; b being placed in the diagram 

 upon the sacral projection. By measuring in 

 eight sacra, the distances hetween the upper 

 extremities of the auricular facets on each side, 

 at the point d, marking them off on paper, 

 and then taking the distances in like manner 

 at c, which corresponds to the lateral notch 

 opposite the second sacral vertebra (see fig. 

 78. A, b.), I found the line d, c to coincide 

 pretty nearly with the mean direction of the 

 superior vertical limb, and with the superior 

 half of the central curved groove d, e ; and to 

 fall in a plane which was inclined to the one on 

 the opposite side, so as to form with it an 

 angle varying from 15 to 25, and in all cases 

 directed downwards. 



Now, the sacrum is so placed in regard to 

 the cotylo-sacral arch, and the line of pressure 

 from above, that the angle formed by the 

 surfaces of the base and anterior face is the 

 narrowest point of a rapidly-expanding arti- 

 cular wedge placed antero-posteriorly. This 

 is better seen by a lateral view of the auricu- 

 lar facet, with the bone in the natural oblique 

 position, as in Jig. 88. A. The facet will 

 then be seen to have its angular projection 

 pointing downwards and forwards in the 

 direction of the cotylo-sacral arch in the line 

 a b, and its two limbs diverging so as to pre- 

 sent a broad surface of articulation with the 

 ilia in the lines b d, be, forming with d e, 

 the triangular " voussoir " d b e. The depth 

 of the keystone is the greatest distance 

 between the anterior border of the superior 

 limb, d, and the inferior extremity of in- 

 ferior limb, e (fig. B), and is about 2^ inches 

 in the adult male. The wedge shape formed 

 by them is well seen in the posterior view of 

 a transverse section along the sacral axis, as 

 in fig. B, where the lines a c, b c show the 

 obliquity of the wedge, and form an angleaci, 

 of from 20 to 35. In a direction downwards 

 and backwards, then, the sacrum has, like an 

 artificial " voussoir," or key stone, its broad end 

 directed upwards towards the point of pressure. 



But, as Cruveilhier has justly observed, 

 forces acting in the curve of the lumbar 

 vertebrae are partly counteracted by the 

 elastic spring-like yielding of the lumbar and 

 sacro-lumbar fibro cartilages ; and by the 

 lumbar curve they are, at the same time, di- 

 rected backwards as well as downwards (viz., 

 at first in the direction of the linerf, f (fig. 

 88. A), and then in that of / e, which latter, 

 produced to meet the vertical line d, b, at g, 

 forms an angle d g e, of about 1 17, coinciding 

 with the average sacro-vertebral angle in the 

 direction of the sacral axis), thus tending to 

 drive the broad end of the sacral " voussoir " 

 between the narrower iliac intervals ; and 

 so, in relation to the direction of the prin- 

 cipal forces acting on the pelvic arch, the 

 sacrum becomes a true keystone. 



Another arrangement which would tend, 

 from the obliquity of the bone, to counteract 

 any forward displacement, is the sudden in- 

 version of tha vertical sacral wedge at the ex- 

 tremity of the lower limb of the auricular 

 surface (fig. B, e), opposite the third sacral 



bone, at which point, we have mentioned in 

 the description of the sacrum, the anterior 

 surface becomes suddenly broader from above 

 downwards ; so that here the sacrum by an- 

 other "joggle" again bites on the iliac. A 

 third disposition preventive of this displace- 

 ment has been pointed out by Mr. Ward, in 

 the superiority of breadth of the posterior 

 over the anterior surface of the sacrum, op- 

 posite the point c. (fig. A.), the middle of the 

 inferior articular limb in many instances. 



Behind and above the angular projection 

 on the sacral facet is an elongated depression 

 or groove, which passes along the centre of 

 both limbs of the auricular surface, and re- 

 ceives a 'reciprocal elevation on the iliac 

 articular surface (fig. 89./) Now this ridge 

 on the iliac surface evidently bites in its turn 

 on the sacrum, and presents another obstacle 

 to anterior displacement in the superior limb, 

 as well as to downward displacement at the 

 inferior limb. The surfaces thus applied to 

 each other, being so curved, give a greater 

 extent of apposition than if they were plane, 

 and, at the same time that they allow of a 

 limited yielding of the sacrum to pressure, 

 keep the surfaces continually in contact. And 

 we shall find that, although the general shape 

 of the articular surface is rendered angular by 

 the "joggle" b (fig. 88. A), the groove and 

 corresponding iliac ridge form a regular cres- 

 centic curve, or segment of a circle d,f, e, 

 of which, in fact, the central internal projec- 

 tion of the tuberosity of the ilium above at 

 a is the centre. Now it is to this prominence 

 that the powerful deep posterior and interos- 

 seous sacro-iliac ligaments are mainly fixed 

 above ; and it is by being suspended by and 

 moving on them in the radius a,f, that the 

 sacrum slides on the ilia downwards and back- 

 wards in the direction of this groove on the 

 reception of force from above. 



That this motion, though limited, does 

 take place, and in this direction, may very 

 readily be proved, on the detached pelvis, by 

 striking directly downwards on the upper 

 extremity of three or four lumbar vertebrae 

 cut off with it. The impulse will be almost 

 entirely felt at the tip of the coccyx, in a 

 direction upwards and backwards. That 

 portion of it which is directed immediately 

 downwards is checked by the powerful liga- 

 ments above mentioned, and is but little felt 

 at the sacral promontory. 



If a section of the whole pelvis, in the 

 direction of the cotylo-sacral arch is made, 

 as in the next figure, a very important element 

 in the mechanism of the sacro-iliac articula- 

 tion is brought to view ; viz., the deep pos- 

 terior and interosseous sacro-iliac ligaments, 

 (d, e.) These ligaments are continuous one 

 into the other, becoming shorter down wards, as 

 the distance between the bones becomes less. 

 They narrow also antero-posteriorly, so as 

 finally to be received into the retiring angle 

 formed by the limbs of the articular facet, at 

 which point they are seen in the transverse 

 section in the sacral axis in fig. 88. B, /. 

 They are attached, externally, to the central 



