THE SPINAL CORD. 501 



arise from nerve-cells in the gray matter. They undergo descending 

 degeneration when divided, but not after division of the dorsal roots. 



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 ventral and dorsal roots. 



The ventral roots are the axons of various groups of nerve-cells situated 

 in the anterior horns of the gray matter. From their origin these axons 

 pass almost horizontally forward through the anterior column in three 

 distinct bundles. After emerging from the cord they curve downward and 

 backward to join the dorsal roots. 



The dorsal roots are the centrally directed axons of nerve-cells in the 

 spinal ganglia. After entering the cord they divide into two main groups, 

 a lateral and a mesial. A portion of the lateral group enters the posterior 

 horn directly through the caput cornu; the other portion turns upward and 

 runs through Lissauer's tract and ultimately enters the posterior horn. The 

 mesial group passes into the postero-external column (Burdach), where 

 the fibers divide into descending and ascending branches. The former 

 probably constitute the comma tract, the terminal branches of which sur- 

 round cells in the gray matter; the latter (ascending) cross the column 

 obliquely and enter the postero-internal column (Goll), in which they pass 

 upward to terminate around the cells of the nucleus gracilis of the same side. 

 As these root fibers pass up and down the cord, collateral branches are 

 given off which enter the gray matter at successive levels and come into 

 physiologic relation with the cells of Clark's vesicular column on the same and 

 opposite sides and with the cells of the anterior horn. 



The peripherally directed axons of the nerve-cells in the spinal nerve 

 ganglia become associated with the axons of the ventral roots and together 

 they pass as a spinal nerve to peripheral organs. 



The ventral root axons are distributed to skeletal muscles, blood-vessels, 

 glands and viscera. The dorsal root axons are distributed to skin, mucous 

 membranes, and muscles. The classification of the nerve-fibers in the 

 ventral and dorsal roots in accordance with the functions they subserve 

 will be found on pages 97, 98. 



Though both the efferent and afferent fibers of the spinal nerves are 

 directly connected with nerve-cells in the spinal cord, they are also indirectly 

 connected by efferent and afferent nerve-tracts with the cerebral cortex. 



Experimentally, it has been determined that the anterior or ventral 

 roots contain all the efferent fibers, the posterior or dorsal roots all the afferent 

 fibers. The proofs in support of this view are as follows: 



Stimulation oj the 'ventral root fibers produces: 



1. Tetanic contraction of skeletal muscles. 



2. Discharge of secretions from glands. 



3. Variations in the degree of the contraction, the tonus, of the muscle 



walls of the peripheral arteries either in the way of augmentation 

 or inhibition. 



4. Variations in the degree of the contraction, the tonus, of the muscle 



walls of certain viscera either in the way of augmentation or in- 

 hibition. 



