THE SPINAL CORD 239 



representing the anterior and posterior cornua of gray mat- 

 ter for that side, and being connected to the corresponding 

 portion of the other side by the commissures embracing the 

 central canal. The anterior cornua are shorter and thicker 

 than the posterior. From these issue the anterior and pos- 

 terior roots respectively of the spinal nerves. The cells are : 

 (i) Those in the anterior cornu; (2) those in the posterior 

 cornu; (3) those in the lateral aspect of the gray matter; 

 (4) those at the inner base of the posterior cornu (Clarke's 

 vesicular column). 



The gray substance is made up of cells with, of course, 

 the usual neuroglia and blood-vessels. The cells in the an- 

 terior cornua are large in size and possess a greater number 

 of poles than those in the posterior cornua ; from their con- 

 nection with the anterior (motor) spinal nerve roots they 

 are called motor cells in contradistinction to the sensory 

 cells in the posterior cornua which are connected indirectly 

 with the posterior (sensory) nerve roots. 



Degeneration. Nerve fibers when separated from the 

 cells of which they are outgrowths degenerate. Fibers have 

 been said to degenerate in the direction in which they carry 

 messages, but this is by no means always so. For instance, 

 the parent cells for the fibers of the posterior spinal roots 

 are in the ganglia on those roots near the cord, and section 

 of the root beyond the ganglion causes degeneration of its 

 fibers peripherally which is in the opposite direction to the 

 passage of impressions in them. Section of the posterior 

 root between the ganglion and cord is followed by centripetal 

 degeneration, and there is no centrifugal degeneration. The 

 anterior spinal root fibers are outgrowths of cells in the an- 

 terior cornua of gray matter. Section of this root anywhere 

 occasions centrifugal degeneration (Fig. 74). 



Arrangement of the White Substance. It is scarcely 

 necessary to state that the white substance of the cord con- 

 sists of nerve fibers with their usual accompaniments. It is 

 external to the gray. The fibers are medullated, but have 

 no sheath of Schwann. 



