362 



PHYSIOLOGY 



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 



FIG. 158. Cells from the oculo-motor nuclei thirteen days after section 



of the nerve on one side. 



a, cell from healthy side ; b, cell from side on which nerve was 



divided. (FLATATI.) 



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 of! those impulses by which their activity is 

 normally excited. Here we have a lesion applied to one neuron 

 exciting 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 twofold 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, however, pro- 

 portional 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 continued directly upwards to the brain or are con- 

 nected with cells which themselves send their axons up to the brain 



