616 PHYSIOLOGY. 



gray matter, midway between the anterior and posterior horns. It 

 lies in what is known as the lateral horn. It is the spinal origin of 

 the great sympathetic. A part of the posterior root-fibers are said 

 to end in these columns. From this as a source fibers pass into the 

 column of Goll and the direct cerebellar tract; others pass into the 

 columns of Burdach and Gowers. 



To the degenerative changes within the cells of the column of 

 Clarke have been attributed the vasomotor troubles of paralysis agi- 

 tans. Sclerosis of the lateral columns explains the exaggerated 

 trembling in the reflexes. 



The fibers of the cells of the gray matter form a spongy sub- 

 stance which unites the two halves of the gray axis of the cord to one 

 another. This, the gray commissure, passes in front of and behind 

 the central canal of the cord. 



Neuroglia. The neuroglia of the gray matter has a structure 

 analogous to that of the neurog'ia of the white substance of the cord. 

 It is found in particular abundance at the extremity of the posterior 

 horns (gelatinous substance of Rolando) and at the periphery of the 

 central canal. 



The Central Canal. This is a canal of very fine caliber located 

 within the center of the gray commissure. It transverses the entire 

 length of the cord, and, at the level of the nib of the calamus scrip- 

 torius, is continuous with the fourth ventricle; by means of the. latter 

 it communicates with the ventricles of the brain. 



The wall of this canal, known as the ependyma, is composed 

 from within outward of: (1) a ciliated epithelium, (2) an amor- 

 phous basal membrane, and (3) a substratum of neuroglia which 

 unites the wall of the canal to the body of the cord. The canal is 

 flanked on each side by a longitudinal vein; the two constitute the 

 central veins. 



Systemization in the Spinal Cord. The spinal cord may be con- 

 sidered as formed of a series of segments superposed. They are 

 metameres corresponding to each pair of spinal nerves. Each one of 

 these is a complete center, being supplied with nerve-cells and motor 

 and sensory nerves. Each one is different from its neighbor, since it 

 innervates a particular area of the surface of the body, whether it be 

 tactile surface or muscular group. 



The nerve-cells are grouped in motor and sensory fiVds. They 

 are all in perfect communication with one another by reason of 

 numerous fibers; some are longitudinal (longitudinal commissures) 

 which unite the various levels of the cord; others are transverse 



