346 HUMAN ANATOMY. 



functional importance of these pelvic contractions should be studied in connection 

 with the mechanism of labor. 



The pelvis may be deformed as a result of morbid conditions in other parts of 

 the skeleton. A. lateral curvature of the lumbar spine to the left may thus be ac- 

 companied not only by the usual compensatory dorsal curve to the right, but by a 

 curve in the latter direction in the sacrum, the upper margin of which will be higher 

 on the right side than on the left. Even the corresponding rotation will take place, 

 and the ala on the concave side will be rotated forward, as are the transverse pro- 

 cesses of the dorsal vertebrae. 



Irregularity in the lengths of the lower limbs may produce a similar curve in 

 the sacrum. 



In both cases the whole pelvis will be tilted laterally, the iliac crest being 

 higher on the convex side of the sacrum. It has been suggested that this continu- 

 ation of a spinal curvature into the sacrum is sometimes a cause, and not a result, of 

 the obliquely contracted pelvis described above, with which it is often associated, 

 but which is regarded as congenital in its origin. 



Humphry, after describing the ring of the pelvis as heart-shaped, and calling 

 attention to the wide arch with a flattened or depressed centre of the upper or 

 posterior half, and the greater curve with flattening at the ilio-pectineal regions 

 of the lower or anterior half, says, ' ' It results from this configuration of the pel- 

 vic ring that it is weakest at five points, — viz., at or a little external to both sacro- 

 iliac synchondroses, at the symphysis pubis, and midway between the latter and 

 the acetabula. Hence fractures, whether from falls, blows, or foreign bodies pass- 

 ing over the pelvis, are most frequent at these points." 



In studying the clinical effects of traumatism applied to the pelvis, it is helpful, 

 however, to consider it with reference to its various functions, — i.e., {a) as inter- 

 posed between the vertebral column and the lower extremities as a weight-carrier ; 

 (J)) as a means of providing for the motions of the trunk upon the lower limbs and 

 of affording advantageous points of attachment for the muscles which effect that 

 motion ; (r) as a bony protection or receptacle for the abdominal and pelvic 

 viscera. 



I. If it is viewed as a bony ring between the spine and the thigh bones, 

 intended to transmit the weight of the head and trunk from the former to the latter, 

 the pelvis may be regarded as made up of two main arches, — one which is in use 

 when standing, the other when sitting. The sacrum is the point of union of both 

 these arches, — one, the femoro-sacral (Morris), extending from the acetabulum 

 through the strong thickened mass of bone indicated on the inner surface by the 

 •upper third of the ilio-pectineal line to the sacrum through the sacro-iliac joint ; the 

 other, the ischio-sacral, extending from the tuber ischii through the strong bony 

 mass at the posterior edge of the acetabulum to the same point. These are the 

 essential weight-carrying portions of the pelvis. 



Although Cunningham says that, as the sacrum narrows towards its dorsal sur- 

 face, and is really suspended from the iliac bones by the posterior sacro-iliac liga- 

 ments, it cannot be considered as the keystone of an arch, yet the union between 

 the sacrum and the ilia is so close by virtue of these powerful ligaments, of those 

 upon the anterior aspect, and of the reciprocal irregularities of the sacro-iliac articu- 

 lar surfaces, that the objection, though technically correct, is clinically a theoretical 

 one only. In describing the mechanics of the remaining or accessory portions of 

 the pelvis, regarded as a weight-carrier, Morris calls attention to the fact that when 

 much strength is essential in an arch, it is often prolonged into a ring so as to form 

 a counterarch, — i.e., the ends of the arch are tied together to prevent them from 

 starting outward. A portion of any weight to be carried by the arch is thus con- 

 veyed to the centre of the counterarch, and borne in what is called the sine of the 

 arch. In the pelvis "the body and horizontal rami of the pubes form the tie or 

 counterarch of the femoro-sacral, and the united rami of the pubes and ischium the 

 tie of the ischio-sacral arch." The ties of both arches are united in front at the 

 symphysis pubis, which, like the sacrum, is common to both arches. 



It can now be understood how and why a fall upon the feet, or a crush either 

 antero-posterior or lateral in direction, though such dissimilar accidents, are so apt to 



