PSOAS ABSCESS. 7 



The psoasmagnus muscle arises from the lower border of the body 

 of the last dorsal and the sides of the bodies of the lumbar vertebrae 

 and the corresponding transverse processes. It is covered with 

 the iliac fascia, the part over the psoas being thinner than that 

 over the iliacus. Above, this fascia becomes attached to the 

 internal arcuate ligament ; on the outer side, to the ventral layer 

 of the lumbar fascia ; on the inner side and below, it is attached 

 to the margin of the sacrum and the brim of the true pelvis. 



Tuberculous material formed in connection with disease of the 

 lower dorsal vertebrae cannot easily pass forwards, owing to the 

 presence of the anterior common ligament, and it cannot readily 

 pass backwards into the spinal canal, owing to the posterior 

 common ligament ; hence it is forced to take a lateral course and 

 gravitate downward in the posterior mediastinum until it reaches 

 the lowest confines of this space. Here it will be directed by the 

 last rib and the twelfth dorsal vertebra behind, and the arcuate 

 ligament in front, into the sheath of the psoas muscle, this being 

 the line of least resistance. 



Tuberculous material formed in connection with the lumbar 

 vertebrae can pass directly outwards into the substance of the 

 psoas along the line of its origin from these bones. 



Whichever way the caseous material finds its entrance into the 

 sheath of the psoas, once there it will tend to invade the whole 

 length of the muscle, and passing behind Poupart's ligament, will 

 enter the thigh. Here it will be at first posterior to the femoral 

 vessels, and subsequently internal to them. The matter as a rule 

 passes to the surface either to the outer side of the femoral 

 vessels immediately below Poupart's ligament, or to the inner 

 side of the upper part of the thigh, below and internal to the 

 position in which a femoral hernia is seen. 



In some cases fluctuation can be readily obtained between the 

 abdominal and thigh portions of the swelling, though commonly 

 the communication is so small that this definite sign is unobtain- 

 able. Moreover, the cavity, in addition to fluid, may contain a con- 

 siderable proportion of solid contents in the form of caseous masses, 

 which greatly tend against the ready production of fluctuation. 



