594 NERVOUS SYSTEM. 



lateral bundle go to the crossed pyramidal tract in the lateral column of the 

 same side and pass upward to decussate at the medulla oblongata. 



The fibres of the columns of Tiirck and the crossed pyramidal tracts are 

 the only fibres of the cord which are known to convey motor impulses from 

 the brain. Destruction of certain parts of the brain produces descending 

 secondary degeneration of these fibres. 



It is probable that fibres arise from the cells of the gray matter of the 

 cord, which connect these cells with each other and are concerned in cer- 

 tain reflex phenomena involving the action of the cord alone. These fibres 

 are in the anterior fundamental fasciculi, the anterior radicular zones and 

 the mixed lateral columns. They present no secondary degeneration. 



The fibres of the posterior roots of the spinal nerves pass to the small, 

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

 nected by branching processes with branching prolongations of these cells. 

 Processes from these cells pass to the gray commissure and decussate around 

 the central canal, conducting sensory impressions to the brain, in the gray 

 matter of the opposite side of the cord. The sensory conductors therefore 

 decussate all along the cord. Some of the fibres go to the columns of Goll 

 and pass upward to and are continuous with the funiculi graciles of the 

 medulla oblongata. Fibres also pass to the direct cerebellar fasciculi and a 

 few, perhaps, to the columns of Burdach, to go upward to the cerebellum. 

 Section of the cord produces ascending secondary degenerations in the col- 

 umns of Goll and the direct cerebellar fasciculi. Fibres originating in the 

 nerve-cells of the posterior cornua pass in and out, along the cord, and con- 

 nect the cells with each other. These may properly be called longitudinal 

 commissural fibres. They probably constitute the greater part of the col- 

 umns of Burdach and they present no secondary degeneration. 



GENERAL PROPERTIES OF THE SPINAL CORD. 



As regards 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 b.y direct muscular contraction follow- 

 ing 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 carefully guard against an extension 

 of the current beyond the points which it is intended to stimulate. Some 

 physiologists regard the cord as absolutely inexcitable and insensible, both on 

 its surface and in its deeper portions. With this view, it is supposed that 

 parts of the cord will conduct 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. 



The results of the observations of Van Deen, Brown-Sequard, Schiff and 

 others, were simply negative ; but the positive results obtained by Longet, 

 Fick, Vulpian and those who regard parts of the cord as excitable and sen- 

 sible, show that certain of the columns react under direct stimulation. 



