THE BONY PELVIS. 



491 



infringe on the cavity of the pelvis, and it is to obviate this that a counter arch is 

 introduced. This secondary arch is formed by the rami and bodies of the pubic 

 bones, and passes anteriorly from one acetabulum to the other on the opposite side. 

 It is much weaker than the primary arch (Fig. 491)- 



Ischio-sacral Arch. In sitting, the pelvis, viewed laterally, is in much the 

 same position as in standing, being in both almost vertical and beneath the spinal 

 column. The thighs, however, are horizontal and the bulk of the weight is sup- 

 ported by the tuber ischii. From the keystone or sacrum the weight is transmitted 

 through the ilium and body of the ischium to the tuberosities on each side. This 

 primary arch is strengthened by the secondary arch formed on each side by the ramus 

 of the ischium and the descending ramus and body of the pubis. Notice that this 

 likewise is weaker than the primary arch (Fig. 492). 



FRACTURES OF THE PELVIS. 



The flaring wings or alae of the ilium are not infrequently fractured by direct 

 violence. The line of fracture is usually transverse (Fig. 493). The displacement 

 is slight on account of the muscular attachments of the iliacus muscle inside and the 

 glutei muscles outside. The anterior third of the crest of the ilium is subcutaneous 

 and prominent, hence by manipulating it crepitus can usually be elicited and the diag- 

 nosis made. Recumbency and the support afforded by adhesive plaster is all the 

 treatment necessary. 



The more serious fractures, however, are those of the true pelvis. The pelvic 

 cavity is somewhat heart-shaped; the sacrum projects anteriorly and is so strong that 

 it is rarely fractured. At or just outside of the sacro-iliac joints is, however, one 

 weak point; between the acetabulum and the 

 pubes, through the rami of the pubes and 

 ischium and thyroid foramen, is another; and 

 at the symphysis is a third. 



The most frequent fracture passes through 

 the pelvic ring twice, anteriorly through the 

 rami of the pubis and ischium and thyroid 

 foramen and posteriorly through or just ex- 

 ternal to the sacro-iliac joint. Whenever a 

 fracture of the pelvis is suspected, search for 

 this fracture. Examine the rami of the pubes. 

 Pressure made along Poupart's ligament just 

 external to the spine of the pubes will usually 

 reveal a tender spot and may elicit crepitus. 

 A digital examination through the rectum or 

 "vagina may likewise indicate the site of the 

 fracture. The bladder is frequently wounded, the rectum almost never and the 

 urethra rarely. 



The symphysis, while comparatively a weak part is rarely the site of injury. In 

 childbirth the attachment of the pubes to each other becomes relaxed and a slight 

 physiological separation occurs. 



FIG. 493. Diagram illustrating fracture of 

 the pelvis; one fracture is seen passing through 

 the ilium; the other passes through the sacro-iliac 

 articulation posteriorly and the thyroid foramen 

 anteriorly. 



THE ATTACHMENT OF THE LOWER EXTREMITIES TO THE 



TRUNK. 



The human body usually occupies one of three positions: standing, sitting, or 

 lying. The functions of the lower extremity are to afford support to the body and 

 accomplish locomotion, therefore any disturbance of the normal relation of the extrem- 

 ities to the trunk interferes with the carrying out of those functions and proper sup- 

 port is not given and locomotion is imperfect. In such cases the positions assumed 

 in standing, sitting, and lying are abnormal, often to an extent sufficient to constitute 

 serious deformities, and locomotion, as in walking or running, is seriously impaired 

 or rendered impossible. 



