XI SPINAL CORD AT VARIOUS LEVELS 469 



m-xlullated) axis cylinders ; but intermixed with these 

 are a certain number of very fine meduUated nerve-fibres, 

 and a few large medullated fibres. 



The Differences in Structure of the Spinal Cord at 

 Various Levels. — These differences show themselves most 

 conspicuously with respect to (i), the shape of the grey 

 matter at various levels (ii), the position of the chief 

 groups of nerve-cells in the grey matter, and (iii), the 

 amount of white matter relatively to the grey matter at 

 each level. The cord is widest in the cervical region, 

 smallest in the thoracic (dorsal) region, and widens out 

 again in the lumbar region. The chief structural differ- 

 ences to which we have alluded are very clearly indicated 

 in Figure 150, which represents sections, drawn to scale, 

 of (half) the spinal cord at the level of A the sixth 

 thoracic (dorsal), B the sixth cervical, and C the third 

 lumbar spinal nerves respectively. 



The Structure of a Spinal Ganglion. — A spinal 

 ganglion is, as we have said (Fig. 144, Gn.), an elongated 

 swelling on the posterior roots of the spinal nerves. In a 

 longitudinal section it is seen to consist of an external 

 sheath of connective tissue which encloses groups of large 

 nerve cells, of which the largest group lies at its outer 

 side. The nerve fibres which enter the distal end of the 

 ganglion on their way to the spinal cord pass in bundles 

 in between the groups of nerve cells, and a certain amount 

 of connective tissue with accompanying blood-vessels and 

 lymphatics, also passes in amongst the nerve cells and 

 nerve fibres. Each nerve cell (Fig. 151, consists like a 

 nerve cell of the spinal cord, of a large nucleus, with a 

 nucleoius, and of a cell body ; but the cell body is, in most 

 cases at all events, prolonged into one process only, so 

 that the whole cell is pear-shaped. This process 

 soon acquires a medulla and a neurilemma ; it thus 

 becomes an ordinary meduUated nerve fibre, which 

 then divides into two fibres, one of which may be traced 

 into the nerve trunk, and the other along the posterior 



