CHAP, i.] THE SPINAL CORD. 879 



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 super- 

 ficial layer of white matter is established. We have already 

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

 565 ; 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. 

 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 



V IV III II I V IV III II I X,l XI X IX VIII VII VI V IV III II I VIII VII VI V IV 



FIG. 101. DIAGRAM SHEWING THE UNITED SECTIONAL AREAS OF THE SPINAL NERVES, 



PROCEEDING FROM BELOW UPWARDS. 



In this as in the succeeding figures 102 3, 5, 6, 7, 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 successively the sacral, lumbar, thoracic 

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



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

 some such figure as that shewn in Fig. 101 ; 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 

 regular. It must be understood however that the dimensions of 

 the areas would not be the same in the two cases. The sectional 

 area of the white matter at the top of the cervical region, though 

 greater than anywhere lower down, is far less than the united 

 sectional area of all the nerves below that level. The white 

 F. 56 



