CHAP, i.] THE SPINAL CORD. 887 



confirmed, (though these diagrams must not be strained to carry 

 detailed inferences,) by the sudden increase of the lateral column 

 above the lumbar swelling, as if the large mass of nervous 

 mechanism for the lower limbs concentrated in 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 explains the change of form in the general outline of the 

 transverse section of the cord which is observed in passing upwards 

 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 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. 106, 107. By 

 comparing the series of transverse sections given in Fig. 104 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 posterior 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 increase 

 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. 



