THE SPINAL MEDULLA 527 



thread, termed the filum terminate. The subarachnoid space is 

 continuous with the subarachnoid space in the skull, and it 

 extends downwards to the second sacral vertebra. Below that 

 point the arachnoid covers the filum terminale. The dura 

 mater spinalis consists of a serous layer only, and is not adherent 

 to the walls of the vertebral canal, to which it is only connected 

 by the spinal nerves. The subdural space is only continued 

 downwards to the second or third sacral vertebra, and, below 

 that level, the dura mater spinalis is closely applied to the filum 

 terminale, which eventually blends with the periosteum on the 

 back of the coccyx. 



Lumbar Puncture. The lower part of the sheath formed by 

 the arachnoid and dura mater contains only the lumbar and 

 sacral nerve-roots and the filum terminale, which form a leash, 

 termed the cauda equina. In lumbar puncture, advantage is 

 taken of the absence of the spinal medulla from the lumbar part 

 of the vertebral canal to introduce an exploring needle into the 

 subarachnoid space. The operation may be performed in order 

 to withdraw some of the cerebro-spinal fluid for examination, 

 or to inject drugs for the purpose of inducing anaesthesia. 



The space between adjoining laminae is greatest in the lower 

 lumbar region, and on that account either the interval between 

 the third and fourth or that between the fourth and fifth lumbar 

 vertebrae is selected. The patient is placed, preferably, in a 

 sitting posture with the trunk strongly flexed, since the interval 

 between the laminae is thus slightly increased. The interval 

 between the fourth and fifth lumbar vertebrae corresponds to the 

 line joining the highest points on the two iliac crests, and the 

 site of the puncture lies on this line half an inch from the median 

 plane. A fine exploring needle, fitted with a stilette, is thrust 

 forwards and slightly upwards and medially through the skin, 

 fasciae, and sacro-spinalis. After passing through the muscle 

 the needle pierces the ligamentum flavum, which connects the 

 laminae ; the ligament can be readily recognised by the resistance 

 that it causes. The needle then enters the vertebral canal and, 

 piercing the dura mater and arachnoid, gains the subarachnoid 

 space, at a distance of about two inches from the surface. If the 

 instrument has reached the subarachnoid space, withdrawal of 

 the stilette will be followed by the escape of cerebro-spinal fluid. 

 It is apparently immaterial whether the fluid withdrawn is 

 obtained from the subdural or from the subarachnoid space. 



When the operation is performed for the purpose of inducing 



