Ch*p. xvi.] PSOAS ABSCESS. 293 



external iliac vessels, the lymphatics and the ureters 

 are outside the iliac fascia, and rest upon its abdo- 

 minal surface, while the anterior crural nerves and 

 abdominal parts of the lumbar nerves are within 

 the osseo-aponeurotic space. Thus, the suprafascial 

 abscess may, with little difficulty, reach the thigh, by 

 following the iliac vessels ; while the subfascial 

 collection would pursue the anterior crural nerve. 



A psoas abscess, or abscess within the fascial 

 sheath of the psoas muscle, is usually due to spinal 

 caries, although it may appear independently of that 

 disease. If the lumbar spine be involved, the matter 

 can pass directly into the substance of the muscle, 

 which it will more or less entirely destroy. If the mis- 

 chief be in the dorsal spine, the matter gravitates along 

 the front of the column until it reaches the diaphragm, 

 which it pierces by an inflammatory process. It is 

 now brought into relation with the heads of the psoas, 

 and has to pass through a narrow strait. "It is 

 prevented from enlarging on the fore-part by the liga- 

 merita arcuata, and at the back by the spine and lowest 

 rib ; hence, in order to proceed it has to force its way 

 in the line of the psoas muscle. That, however, can 

 only be done by penetrating into its interior. It ac- 

 complishes this, in the first place, by inserting its most 

 advanced part, like a wedge, between the two origins 

 (viz., from the bodies of the vertebrse, and from the 

 corresponding transverse processes) : it then splits and 

 distends the fibres, so as to form a cavity for the 

 reception of the pus .... and the psoas at length is 

 converted more or less thoroughly into an abscess" 

 (Shaw). The pus, following the muscle, at last reaches 

 the thigh, and usually points, just below the groin, to 

 the outer side of the femoral vessels. 



The abscess, however, often shows much variation. 

 It may avoid the psoas, or leave it when once it has 

 entered it, and make its way into the lumbar region, 



