LUMBAR PUNCTURE 41 



the site for the operation of lumbar puncture, because (i) the 

 site chosen should provide a free flow of the cerebro-spinal 

 fluid ; (2) there is no danger of injuring the delicate spinal 

 medulla; (3) the spaces between contiguous laminae is greatest 

 in this region. There is little danger of injuring the nerve- 

 roots of the cauda equina, as they tend to be pushed aside by 

 the point of the needle. 



The space between the fourth and fifth laminae may be 

 chosen, or the space between the fifth and the sacrum. Some 

 authorities prefer to enter the needle in the posterior median 

 line. In that case, the instrument passes between two con- 

 secutive spines instead of through the interlaminar interval. It 

 must be remembered that the fourth lumbar vertebra lies on 

 the line joining the highest points on the iliac crests. When 

 the interlaminar interval between the fourth and fifth lumbar 

 vertebrae is selected, the needle is inserted about half an inch 

 below this line and rather less than one inch from the median 

 plane, and it is thrust in a forward and slightly medial direction. 

 If the point of the needle meets bone, it must be partially 

 withdrawn and then re-inserted, after some alteration has been 

 made in its direction. When the interlaminar interval is 

 gained, the operator experiences the characteristic resistance 

 due to the strong ligamentum flavum, which fills in the gap 

 between the laminae. In the adult, the instrument will require 

 to be introduced to a depth of about two inches before it 

 reaches the vertebral canal, but in the young child the distance 

 is rather less than one inch. 



In order to separate the laminae as far as possible, the 

 patient should lie on his side with the body fully flexed, while 

 the operation is being carried out. If the interspinous interval 

 is chosen, the needle must be directed forwards and slightly 

 upwards, parallel to the inferior border of the spine. In this 

 case, the strong supra-spinous ligament must first be pierced 

 and the instrument then passes forwards through or by the 

 side of the interspinous ligament, before it enters the canal. 



After the needle has been successfully passed into the canal, 



