CHAP, i.] THE SPINAL COED. 



the transverse section of the cord which is observed in passing 

 upwards from the lower to the higher regions. In the coccy- 

 geal, sacral and lumbar regions the outline, though varying 

 somewhat chiefly owing to the disposition of the grey 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 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-ventral 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 grey matter. 



The cord moreover undergoes along its length a change 

 which is not very clearly indicated in the diagrams Figs. 119, 

 120. By comparing the series of transverse sections given in 

 Fig. 114 it will be seen that the relative 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 region, the position of the central 

 canal is shifted nearer to the ventral surface so that the poste- 

 rior fissure becomes relatively longer, deeper, than the anterior. 

 This shifting goes on through the cervical region up to about 

 the level of the 2nd cervical nerve, where it is arrested by the 

 beginning of the changes through which the spinal cord is 

 transformed into the far more complicated bulb. 



This lengthening of the posterior fissure indicates an in- 

 crease in the dorso-ventral diameter of the posterior columns, 

 and this, not being accompanied by a compensating diminution 

 of the side to side diameter, shews in turn that the posterior 

 columns undergo an increase in passing upwards. From this 

 we may add to the provisional conclusion just arrived at with 

 regard to the lateral columns, the further conclusion that some 

 part of the posterior columns also is concerned in transmitting 

 impulses, in a more or less direct manner, between the various 

 regions of the cord below and the brain above. The anterior 

 columns do not increase in the same marked manner, though 

 over and above the increase due to the lumbar and cervical 

 swellings, a continued increase may be observed especially in 

 the upper cervical region ; it is in this upper region that the 

 direct pyramidal tract is best developed. 



