SOCIETY OF THE UNIVERSITY OF ABERDEEN. 5 



the Dura and the Arachnoid. They may practically be regarded as one, 

 since the subdural space is only a very narrow chink, a potential space just 

 moistened with fluid. These two sheaths forming the sac almost fill up the 

 spinal canal, being separated from the bony wall and ligaments only by some 

 fatty and areolar tissue and by a plexus of veins. They are continued down 

 beyond where the cord stops, and indeed the subarachnoid space is largest 

 in the lumbar region, being here about f in. in breadth and J in. in depth 

 antero-posteriorly. Opposite the 5th lumbar vertebra, the sac begins to taper 

 and ends opposite the 2nd or 3rd Sacral spine. Thus a large proportion of 

 the cerebro-spinal fluid is contained in the lumbar portion of the spine. The 

 space communicates superiorly with the cranial subarachnoid space and with 

 the ventricular system. Lumbar puncture itself is usually performed between 

 the 3rd and 4th lumbar vertebras, and still commonly as Quincke described 

 it, that is, a little lateral to the middle line. 



Having mentioned these facts, we proceed to the observations made on 

 the dissecting-room subjects. The lumbar curve, convex forwards, makes the 

 spinous processes lie very closely together. These processes are of moderate 

 length, about 1 in. to 1^ in. (2 - 5 to 3 - 2 cm) in their middle, flattened from 

 side to side, quadrangular in shape, with broad relatively flat extremities in 

 the case of the upper two or three, less broad and nearly always rounded 

 in the lower two or three. The average vertical breadth of the tip of the 

 1st lumbar spine is over f in. (2 - l cm), and in the case of the others steadily 

 diminishes till the 5th is reached, where the average is only about J in. (1'4 cm). 

 For the identification of the lumbar spinous processes, it is unreliable to 

 count down from the 7th cervical spine, because the 6th cervical spine may 

 be nearly as prominent as the 7th. So also, counting from the 12th rib 

 cannot be trusted, because the rib may not project beyond the outer margin 

 of the Erector Spinae and may be missed altogether. The best landmark is 

 the line joining the highest points of the two iliac crests. This is pretty 

 constant, regardless of age and sex, in crossing the 4th lumbar spine. German 

 writers give different levels. In the case of 15 out of 17 bodies examined 

 in the dissecting room, the line crossed the 4th spine, generally in the middle, 

 and in the other two bodies the space between the 3rd and 4th spines. It 

 is quite unreliable to judge of the level of the iliac crests from skiagrams, 

 since in these the lamp is usually placed perpendicularly over the umbilicus. 



