690 THE FEATURES OF DIFFERENT REGIONS. [Book in. 



the cord increases therefore from below upwards, but not regu- 

 larly, the irregularity being due to the lumbar and cervical 

 swellings. 



The extremity of the filum terminale is said to consist en- 

 tirely of neuroglia closely invested by the membranes, even the 

 central canal being absent. A little higher up the central canal 

 begins, and nerve-cells with nerve-fibres make their appearance 

 in the neuroglia ; thus a kind of grey matter covered by a thin 

 superficial layer of white matter is established. We have else- 

 where referred to the peculiar features of the lower end of the 

 conus ; but higher up the canal becomes central and small, the 

 posterior columns are developed, and the grey matter contains 

 more nervous elements and relatively less neuroglia, becomes in 

 fact ordinary grey matter. From thence onward to very near 

 the junction with the bulb, where transitional features begin to 

 come in, the spinal cord may be said to have the general structure 

 previously described. 



The sectional area of the white matter increases in absolute 

 size and on the whole in a steady manner from below upwards. 



II I VIII VII VI V IV III II I 



Fig. 115. Diagram shewing the united sectional areas of the Spinal, 

 Nerves, proceeding from below upwards. 



In this as in the succeeding figures 116 — 17, — 18, — 19, — 20, all of which refer 

 to man, the left-hand side represents the bottom of the cord and the right-hand the 

 top of the cord, the numerals indicating successfully the sacral, lumbar, thoracic 

 and cervical nerves. The several figures are not drawn to the same scale. 



In other words, in a section at any level, the number of longi- 

 tudinal fibres forming the white matter is greater than the 

 number at a lower level, and less than the number at a higher 

 level ; for any difference which may exist in the diameter of the 

 individual fibres is insufficient to explain the differences in the 

 total sectional area of the white matter. If we were to measure 

 in man the sectional area of each of the spinal nerves as it joins 

 the cord, and to add them together, passing along the cord from 

 below upwards the results put in the form of a curve would give 

 us some such figure as that shewn in Fig. 115 ; the area gained 

 by adding together the sectional areas of the nerves increases 

 in a fairly steady manner from below upwards. The curve of 

 the sectional area of the white matter of the cord taken from 

 below upwards would be very similar, but if anything more 



