554 NERVOUS SYSTEM 



the cell-bodies, the connection between them, however, being indirect 

 and not directly continuous. Some neurites from the posterior roots 

 possibly are directly continuous with neurites from the cell-bodies. 



It is probable that fibres pass from the cell-bodies and go to the cerebel- 

 lum in the direct cerebellar fasciculi, the columns of Goll and the ascend- 

 ing cerebellar fasciculi, which show ascending secondary degenerations. 



Certain fibres from the posterior roots of the spinal nerves pass to the 

 cells of the posterior cornua of gray matter of the cord and are con- 

 nected by arborizing processes with ' arborizing prolongations of these 

 cells (synapses). Processes from these cells pass to the gray commis- 

 sure and decussate around the central canal, conducting sensory im- 

 pressions to the brain in the gray matter of the opposite side of the 

 cord. The sensory conductors, therefore, decussate all along the cord. 

 Fibres originating in the nerve-cells of the posterior cornua pass in 

 and out, along the cord, and their synapses connect the cells with each 

 other above and below. These may properly be called longitudinal 

 commissural fibres. They probably constitute the greatest part of the 

 columns of Burdach. 



GENERAL PROPERTIES OF THE SPINAL CORD 



In describing the general properties of the cord, as shown by the 

 effects of stimulus .applied to its exterior or to its cut surface, the 

 term "excitability" will be used to express a property indicated by direct 

 muscular contraction following stimulation of the cord, and sensibility, 

 a property which enables it to receive impressions which produce pain. 

 In exciting different parts of the cord with electricity, it is necessary to 

 guard carefully against an extension of the current beyond the points 

 which it is intended to stimulate. Some physiologists regard the cord 

 as inexcitable and insensible, both on its surface and in its deeper por- 

 tions. With this view, it is supposed that parts of the cord will con- 

 duct motor impulses received from the centres situated above but are 

 not excited by a stimulus applied directly. In the same way, it is 

 thought, parts of the cord will convey sensory impressions received 

 through the nerves but are insensible to direct irritation. Certain of 

 the columns, however, react under direct stimulation. 



In experiments made in 1863 (Flint) on a living dog, the cord 

 having been exposed in the lumbar region and stimulated mechani- 

 cally and with an electric current two hours after the operation, certain 

 positive results were obtained which led to the following conclusions : 



The gray substance probably is inexcitable and insensible to direct 

 stimulation. 



