SPINAL CORD. 189 



3. Vaso-motor centers, giving origin to both vaso-constrictor and 

 vaso-dilator fibers, which are distributed throughout the cord. 

 Though acting reflexly, they are under the dominating influence 



of the center in the medulla. 



4. Sweat -centers are also present in various parts of the cord. 



2. As a Conductor. 



The white matter of the spinal cord consists of nerve-fibers, the 

 specific function of which is, 



1. To conduct nerve impulses from one segment of the cord to 

 another. 



2. To conduct nerve impulses coming from the encephalon to the 

 spinal cord segments. 



3 To conduct nerve impulses coming to the cord through afferent 

 nerves, directly or indirectly to the encephalon. 



Intersegmental Conduction. The spinal cord consists of a series 

 of physiologic segments each of which has a special function and is 

 associated through its related spinal nerve with a definite segment 

 of the body. For the harmonious cooperation and coordination of 

 all the spinal segments it is ^ssential that they should be united 

 by commissural or associative fibers. The cord thus becomes capable 

 of complex and purposive reflex actions. 



Encephalo-spinal, or Motor Conduction. The nerve-fibers which 

 conduct volitional impulses from the brain downward to the an- 

 terior cornua arise in the motor centers of the cerebrum ; they 

 then pass downward through the corona radiata, the internal cap- 

 sule, the inferior portions of the crura cerebri, the pons Varolii, 

 to the medulla oblongata, where the motor tract of each side di- 

 vides into two portions, viz. : 



1. The larger, containing ninety-one to ninety-seven per cent, of the 

 fibers, which decussates at the lower border of the medulla and 

 passes down in the lateral column of the opposite side, and con- 

 stitutes the crossed pyramidal tract. 



2. The smaller, containing three to nine per cent, of the fibers, 

 does not at once decussate, but passes down the anterior column 

 of the same side, and constitutes the direct pyramidal tract, or the 

 column of Tiirck. At a lower level this tract also decussates or 

 crosses over to the opposite side of the cord. 



The fibers of both the crossed and the direct pyramidal tracts come 

 into relation by their terminal branches with the nerve-cells in the 



