CHAP, i.] THE SPINAL COED. 691 



regular. It must be understood however that the dimensions of 

 the areas would not be the same iri 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 

 grey matter. Nevertheless the white matter in passing up 

 the cord appears to receive a permanent addition at the entrance 

 of each nerve. We may infer that each nerve has a representa- 

 tive of itself starting from the level of its entrance and running 

 up to some part of the brain. 



454. The grey matter in contrast to the white matter 

 shews great variations in area along the length of the cord (Fig. 

 116). From the entrance of the coccygeal nerve upwards the 



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



FIG. 116. DIAGRAM SHEWING THE VARIATIONS IN THE SECTIONAL AREA OF THE 

 GREY MATTER OF THE SPINAL CORD, ALONG ITS LENGTH. 



area increases very rapidly, reaching a maximum at about the 

 level of the 5th lumbar nerve. It then rapidly decreases to about 

 the level of the llth thoracic nerve, maintains about the same 

 dimensions all through the thoracic region, and begins to increase 

 again at about the level of the 2nd thoracic nerve. Its second 

 maximum is reached at about the level of the 5th or 6th cervical 



I V IV 111 II I XII XI X IX Vlll VII VI V IV III II I VIII VII VI V IV III >l I 



FIG. 117. DIAGRAM SHEWING THE RELATIVE SECTIONAL AREAS OF THE SPINAL 

 NERVES, AS THEY JOIN THE SPINAL CORD. 



nerve, after which the area again becomes smaller, remaining 

 however at the upper cervical region much larger than in the 

 thoracic region. 



The meaning of these variations becomes clear when we turn 

 to Fig. 117, which shews in a similar diagrammatic manner the 



