STRUCTURE OF THE SPINAL CORD 321 



more pronounced according as the lesion is nearer to the cell (Fig. 157.) If, for instance, 

 an anterior root be divided and three weeks later the animal be killed and sections made 

 of the corresponding segment of the cord and stained with toluidine blue or methylene 

 blue, a striking difference will be observed between the cells of the anterior horn of 

 the two sides of the cord. On the side of the lesion the nucleus of the cells will be 

 somewhat swollen, and may be displaced towards the periphery of the cell. The Nissl 

 granules are no longer distinct, but the whole cell is diffusely stained blue. In some 

 cases this change may go on to complete atrophy of the cell and consequent degenera- 

 tion of the whole of its axon. Generally however the cell gradually recovers, so that 

 six months after the lesion no difference will be observable between the cells on the 

 two sides of the cord. This method must be used with some caution as a means of 

 tracing out the connections of any given neurons in the central nervous system, since 

 it has been shown by Warrington that somewhat similar changes may be produced 

 in the anterior horn-cells by division of the posterior roots, thus cutting off those im- 

 pulses by which their activity is normally excited. Here we have a lesion applied to 

 one neuron causing a histological change in the cell body of another neuron which 

 is next in the chain of the nervous arc. 



The structure of the cord is closely connected with and determines its two- 

 fold function, namely, as a series of reflex centres for the different segments 

 of the body, and as a means of communication between the trunk and limbs 

 and the higher parts of the central nervous system. An examination of the 

 relative area of the white matter at different levels of the cord shows a 

 steady increase from the lower to the upper end. The increase is not how- 

 ever proportional to the number of fibres which enter or leave the cord in the 

 various spinal nerve roots. Of these fibres therefore a certain proportion are 

 destined to serve merely the local segmental reflexes, while others are con- 

 tinued directly upwards to the brain or are connected with cells which them- 

 selves send their axons up to the brain (cells of the columns). All the motor 

 fibres in the nerve roots arise from cells in the spinal cord near the point of 

 origin of the root. Any direct influence of the brain on the motor mechan- 

 isms of the body is therefore effected through the intermediation of the 

 segmental neural mechanisms of the grey matter of the cord. We will 

 consider the function and related structure of the cord in these two aspects : 

 first, as a reflex centre, and secondly, as a conductor of impulses to the 

 higher parts of the central nervous system. 



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