8 PUOCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



to be regarded as practically straight. Bleeding from the venous plexuses 

 lying round the bases of the spinous processes and in the spinal canal between 

 the bones and the meninges is said to be less frequent with puncture in the 

 middle line than in the other method, and certainly the veins tend to lie, 

 though not very symmetrically, in lateral sets. Still it occurs occasionally 

 that after repeated failures to find fluid through the middle line, one succeeds 

 on trying Quincke's lateral puncture. Though trickling away of the fluid 

 is the only reliable sign that the point of the needle has entered the sac, 

 yet it is useful to know how deep the needle has to go. In 17 bodies, the 

 depth from the tip of the spinous process (the landmark least affected by the 

 musculature and the state of nutrition) to the bottom of the spinal canal 

 ranged from 2 in. (51 cm) to 2J in. (6'4 cm), the average being 2 in. (5'5 cm). 

 In dried skeletons and lumbar vertebrae, the range was from If in. to 2 in. 

 (4*3 cm to 5 - l cm) with an average of 4'9 cm. The difference can be accounted 

 for by the thickness of the skin and subcutaneous tissue covering the spinous 

 processes. As the point of the cannula should come to lie in the hindmost 

 part of the sac, the antero-posterior diameter of which may be taken as 

 approximately i in., the needle ought to penetrate 1| in. to 2 in. (4 to 6 cm). 

 In a child the distance is under an inch (2 to 2'5 cm). The width of the 

 lumbar bony canal is f in. to 1 in., averaging nearly 1 in., and the antero- 

 posterior diameter f in., and since the sac fills the greater part of this space, 

 the needle, once it passes the interlaminar opening, can hardly fail to hit 

 the sac. Failure to find fluid may be due to the absence of a Cisterna, at 

 least so low down, but far more frequently to technical errors, such as blocking 

 of the needle by a shred of flesh or by a nerve lying in front, or penetrating 

 either not deep enough or too deep. In the latter case both front and back 

 walls of the sac are perforated, but it does not follow that the progress of 

 the Cannula is stopped at once by bone. It may pass through the posterior 

 common ligament and enter the spongy substance of the lower half of the 

 body of the upper vertebra or the upper part of the intervertebral disc. 

 The Vena Basis Vertebrse and the big transverse veins flowing outwards from 

 it beneath the Posterior Common Ligament is in a position to be punctured, 

 with resulting flow of blood which might be mistaken for blood effused into 

 the subarachnoid space. 



