THE STRUCTURE OF THE SPINAL CORD. 



547 



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 upward. In other words, in a 

 section at any level, the number of longitudinal 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 



FIG. 124. 



XII XI X IX VIII VII VI V IV III II I VIII VII VI V IV III II I 



Diagram showing the United Sectional Areas of the Spinal Nerves, proceeding from Below 

 Upward. In this, as in the succeeding figures, 125, 126, 127, 128, 129, 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 nu- 

 merals indicating successively the sacral, lumbar, thoracic, and cervical nerves. The several 

 figures are not drawn to the same scale. 



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 upward, the results put in the form of a 

 curve would give us some such figure as that shown in Fig. 124 ; the area 

 gained by adding together the sectional areas of the nerves increases in a 

 fairly steady manner from below upward. The curve of the sectional area 

 of the white matter of the cord taken from below upward would be very 

 similar, but if anything more regular. It must be understood, however, 



FIG. 125. 



V IV III II I V IV III 



I XII XI X X VIII VII VI V IV 



VIII VII VI V IV III II 



Diagram showing the Variations in the Sectional Area of the Gray Matter of the Spinal Cord 



along its Length. 



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 matter is not formed by all 

 the fibres from the nerves which join the spinal cord continuing to run along 

 the cord up to the brain ; as we have seen, some at least of the fibres end in 

 the gray matter. Nevertheless, the white matter in passing up the cord ap- 



