chap, xviii.] THE PELVIS. 339 



The sacro-iliac sychondrosis may be the seat 

 of disease. Normally, there is no movement at this 

 joint, but when the ligaments are softened by disease, 

 and effusion occurs between the opposed bones, some 

 movement may be demonstrated. As this joint lies in 

 the line of the great arches of the pelvis, it follows 

 that when inflamed much pain is felt in the part, 

 both when the patient is standing or sitting. When 

 abscess forms it tends to come forwards, owing to the 

 fact that the anterior ligaments are less dense than 

 the posterior. Having reached the pelvic aspect 

 of the joint, the pus may occupy the iliac fossa, or 

 gain the ilio-psoas sheath. Or it may follow the 

 lumbo-sacral cord and great sciatic nerve and point in 

 the thigh behind the great trochanter, or it may be 

 guided by the obturator vessels to the inner side of 

 the thyroid foramen, and ultimately appear at the 

 inner side of the thigh. The abscess may, however, 

 proceed backwards, and point over the posterior aspect 

 of the joint. 



The nerve relations of this joint are important. 

 It is supplied by the superior gluteal, by the lumbo- 

 sacral cord and first sacral nerve, and by the first and 

 second posterior sacral nerves (Morris). The lumbo- 

 sacral cord and the obturator nerve pass over the 

 front of the joint, the former being very closely con- 

 nected with the articulation. It will be understood 

 from these relations that in sacro-iliac disease pain 

 is felt over the sacral region (upper sacral nerves) and 

 in the buttock (gluteal nerve). Much pain is also 

 often complained of in the hip or knee-joint, and 

 along the inner part of the thigh (obturator nerve). 

 In one or two reported cases there has been severe 

 pain in the calf and back of the thigh, with painful 

 twitchings in the muscles of those parts (lurnbo- 

 sacral cord and connection with great sciatic nerve). 

 Dislocation of the sacrum at this joint is prevented by 



