184 HUMAN PHYSIOLOGY. 



(a) The crossed pyramidal tract, occupying the posterior portion 

 of the lateral column, and containing all those fibers of the 

 motor tract which have decussated at the medulla oblongata ; 

 it is composed of longitudinally running fibers, which are con- 

 nected with the multipolar nerve-cells of the anterior cornua. 

 (fr) The direct cerebellar tract, situated upon the surface of the 

 lateral column, consisting of longitudinal fibers which termi- 

 nate in the cerebellum ; it first appears in the lumbar region, 

 and increases in thickness as it passes upward. 

 (c) The anterior tract, lying just posterior to the anterior cornua. 

 3. A posterior column, the portion included between the posterior 

 roots and the posterior fissure, also divisible into two portions : 

 (a) An inner portion, the postero -internal column, or the column 



of Goll, bordering the posterior median fissure, and 

 (&) An external portion, the postero-external column, the column 



of Burdach, lying just behind the posterior roots. 

 The two portions of the posterior column are composed of long and 

 short commissural fibers, which connect different segments of the 

 spinal cord. 



The Relation of the Spinal Nerves to the Spinal Cord. The 



spinal nerves present near the spinal cord two divisions which from 

 their connection with the anterior or ventral and the posterior or 

 dorsal surfaces are known as the anterior or ventral and posterior 

 or dorsal roots. The ventral roots are composed of nerve-fibers which 

 have their origin in the nerve-cells in the anterior horns of the gray 

 matter. The dorsal roots are composed of nerve-fibers which have 

 their origin in the nerve-cells in the spinal ganglia. After entering. 

 the cord some of the posterior or dorsal root-fibers arborize around 

 nerve-cells in the gray matter at the same level ; others pass obliquely 

 upward through the posterior white columns as far as the nucleus 

 gracilis and the nucleus cuneatus around the nerve-cells of which 

 they terminate. 



FUNCTIONS OF THE SPINAL CORD. 



The spinal cord, by virtue of its contained nerve-cells and nerve- 

 fibers, may be regarded as composed of 



1. Independent nerve centers each of which has a special function; 

 and 



2. Of conducting paths by which these centers are brought into 



