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HAND-BOOK OF PHYSIOLOGY. 



of the cord are not alike able to conduct all impressions; and as there are 

 separate nerve-fibres for motor and for sensory impressions, so in the cord, 

 separate and determinate parts serve to conduct always the same kind of 

 impression. 



Experiments (chiefly by Brown-Sequard), point to the following con- 

 clusions regarding the conduction of sensory and motor impressions 

 through the spinal cord. 



It is important to bear in mind that the grey matter of the cord, 

 though it conducts impressions giving rise to sensation, appears not to be 

 .sensitive when it is directly stimulated. The explanation probably is, 



FIG. 320. Diagram of the decussation of the conductors for voluntary movements, and those for 

 sensation: a, r, anterior roots and their continuations in the spinal cord, and decussation at the 

 lower part of the medulla oblongata, m o; p r, the posterior roots and their continuation and decus- 

 sation in the spinal cord; g g, the ganglions of the roots. The arrows indicate the direction of the 

 nervous action; r, the right' side; Z, the left side. 1, 2, 3, indicate places of alteration in a lateral half 

 of the spino-cerebral axis, to show the influence on the two kinds of conduc* resulting from sec- 

 tion of the cord at any one of these three places. (After Brown-Sequard.) 



that it possesses no apparatus such as exists at the peripheral terminations 

 of sensory nerves, for the reception of sensory impressions. 



a. Sensory impressions, conveyed to the spinal cord by root-fibres of 

 the posterior nerves are not conducted to the brain only by the posterior 

 columns of the cord, but pass through them in great part into the central 

 grey substance, by which they are transmitted to the brain (p r, Fig. 320). 



b. The impressions thus conveyed to the grey substance do not pass 

 up to the brain to more than a slight degree, along that half of the cord 

 corresponding to the side from which they have been received, but cross 



