THE SPINAL CORD 553 



been observed and studied, but without developing distinct and positive 

 physiological applications. 



Directions of Nerve-fibres in the Cord. Many of the points in the 

 description of the course and connections of the fibres in the cord are 

 given as probable. Anatomical observations have been somewhat con- 

 tradictory, but many of these have been corrected or verified by follow- 

 ing the paths of degeneration. What are called secondary degenerations 

 are anatomical changes in the nerve-fibres which follow separation of 

 the fibres from the cells which act as their trophic centres, or the 

 centres presiding over their nutrition, these changes being secondary to 

 destruction or degeneration of the centres. 



The fibres of the anterior roots of the spinal nerves, following these 

 fibres inward and upward, pass to the large multipolar motor cells of 

 the anterior cornua of gray matter and have no direct connection with 

 the white columns. Their direction through the white columns of the 

 cord is oblique and slightly upward. Surrounding the nerve-cells are 

 arborescent fibrils (synapses) connected with the fibres that pass up 

 the cord in two bundles, median and lateral. So far as conduction is 

 concerned, these fibres may be regarded as continuous with the fibres 

 of the anterior roots. The fibres of the median bundle pass to the an- 

 terior white commissure, in which they decussate. The bundles then 

 go each one to the column of Ttirck on the opposite side and pass up- 

 ward in the so-called direct pyramidal tracts. The fibres of the lateral 

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

 same side and pass upward to decussate at the bulb. 



The fibres of the columns of Tiirck and the crossed pyramidal 

 tracts are the only fibres of the cord that are known to convey motor 

 impulses from the brain. Destruction of certain parts of the brain 

 produces descending secondary degenerations in 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 by means of synap- 

 ses and are concerned in certain reflex phenomena involving the action 

 of the cord alone. These fibres are in the anterior fundamental fascic- 

 uli, the anterior ground columns and the lateral bundle. They present 

 no secondary degenerations. 



The fibres of the posterior roots of the spinal nerves pass into the 

 white substance of the cord, where they immediately bifurcate, one 

 limb passing downward and the other upward for a certain distance, 

 before they enter the gray matter. The upper limb of the bifurcation 

 is the longer. In their course they send off collateral branches which 

 penetrate the gray matter and break up into arborescent fibrils (synap- 

 ses). These mingle and interlace with similar aborescent fibrils from 



