THE STRUCTURE OF THE SPINAL CORD. 



553 



this region demanded a sudden increase in the number of fibres connecting 

 it with the brain above. 



This more or less continuous increase of the lateral column partly ex- 

 plains the change of form in the general outline of the transverse section 

 of the cord which is observed in passing upward from the lower to the 

 higher regions. In the coccygeal, sacral, and lumbar regions the outline, 

 though varying somewhat, chiefly owing to the disposition of the gray 

 matter, is on the whole circular. In the thoracic region, especially in the 

 upper part, the increase of the lateral columns increases the side-to-side 



FIG. 128. 



y ( V HI || I V IV III II I XII XI X IX VIII VII VI V IV III II I VIII VII VI V IV III 



Diagram showing the Variations in the Sectional Area of the Lateral Columns of the Spinal 



Cord along its Length. 



FIG. 129. 



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



VIII VII VI V IV 



Diagram showing the Variations in the Sectional Area of the Anterior Columns of the Spinal 



Cord along its Length. 



FIG. 130. 



I XII XI X IX VIII VII VI V IV 



Diagram showing the Variations in the Sectional Area of the Posterior Columns of the Spinal 



Cord along its Length. 



diameter so much that the section becomes oval, and in the cervical region 

 this increase of the side-to-side diameter out of proportion to the dorso-ven- 

 tral diameter is very marked. The actual outline of the whole transverse 

 section is, however, determined also to a certain extent by the changes of 

 form of the gray matter. 



The cord, moreover, undergoes along its length a change which is not 

 very clearly indicated in the diagrams (Figs. 129, 130). By comparing the 

 series of transverse sections given in Fig. 127, it will be seen that the rela- 

 tive position of the central canal shifts along the length of the cord. In the 

 sacral and lumbar regions the central canal is nearly at the centre of the 

 circle of outline, and the posterior and anterior fissures are nearly of equal 

 depth. Even in the upper lumbar region, and still more in the thoracic re- 

 gion, the position of the central canal is shifted nearer to the ventral sur- 

 face, so that the posterior fissure becomes relatively longer, deeper than the 



