THE SPINAL CORD 239 



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 anterior 

 cornua are large in size and possess a greater number of poles 

 than those in the posterior cornua; from their connection 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 anterior 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 consists of 

 nerve fibers with their usual accompaniments. It is external to 

 the gray. The fibers are medullated, but have no sheath of 

 Schwann. 



The divisions of the cord already referred to are purely ana- 

 tomical. Physiological and pathological researches warrant the 

 further division of the white substance of the cord into eight 

 columns for each side. The course of all the fibers in the white 



