^^g- 55- Lower End of Spinal Canal in the Adult. 



A Figure combined after a specimen from a man of i8, and several skeletons. 

 On the rigid, bones only, on the left, ligaments and nerves have been drawn 



diagrammatical ly. 



The anatomical relations make it possible to introduce a trocar or a hollow 

 needle from behind, between 2 neural arches, into the dorsal sac, thus enabling 

 one to increase the pressure of the cerebrospinal fluid in the sac, and to draw 

 off some of the fluid for chemical and microscopical examination. This operation 

 — lumbar puncture — has become of vast importance for diagnostic purposes 

 in recent years. 



Not onl\^ does it yield information as to the condition of the fluid around 

 the spinal cord, .but also as to the intra-ventricular pressure, the pathological 

 changes and the presence of bacteria in the cerebral fluid, because these fluids 

 communicate through the Foramen of Magendie at the floor of the 4th Ventricle. 



The lowest portion of the dural sac is selected for lumbar puncture, 

 because the needle cannot, at that point, injure the spinal cord; it meets the 

 Cauda Equina which gets pushed out of the way. The needle should be intro- 

 duced below the 3rd or 4th Lumbar Vertebra. The vertebra may be found by 

 counting the spinous processes downwards from the 7th Cervical Vertebra, or by 

 counting downwards from the attachment of the 12th rib. A far more simple 

 method ■ — (cf. figure) — consists in drawing a line connecting the highest points 

 of the Iliac Crests. This line crosses the middle of the 4th Lumbar Vertebra. 

 Slightl}' above it, therefore, is the spinous process of the 3rd. In children, the 

 needle may be introduced exactlj' in the middle line at the lower border of the 

 spinous process; in adults, it should be introduced Vs'hs inch outside the middle 

 line, because the strong median ligaments offer considerable resistance. 



If one follows the usual rule, introducing the needle at the level of the 

 lower border of the spinous process and then pushing it forwards, upwards, and 

 inwards, one may come on to bone, especially when the spinous process is short, 

 (this was the case in our specimen). It is infinitely better to introduce the needle 

 horizontally inwards. . 



In the figure, "Point for Lumbar Puncture"' indicates the spot on the skin, 

 at which the needle should be introduced and then pushed inwards. 



In children the needle has to enter about one inch, in adults 1Y5 to 

 2^/5 inches. 



This figure also shews the Dural Cul-de-sac at the 2nd Sacral Vertebra. 



The importance of this level has been discussed in Fig. 54, Text. 



