SPINAL CORD. jyj 



2. Genitospinal center, situated in the lower part of the cord. This is a 

 complex center, and comprises a series of subordinate centers for the control 

 of the muscular movements involved in the acts of defecation, micturition, 

 and ejaculation of semen, and of the movements of the uterus during 

 parturition, etc. 



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

 dilatator fibers, which are distributed throughout the cord between the 

 first thoracic and third lumbar nerves. 



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 phys- 

 iologic 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 har- 

 monious cooperation and coordination of all the spinal segments it is essential 

 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 ventral cornua 

 arise in the motor centers of the cerebrum; they then pass downward through 

 the corona radiata, the internal capsule, the inferior portions of the crura 

 cerebri, the pons Varolii, to the medulla oblongata, where the motor tract of 

 each side divides 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 constitutes 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 ventral 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 



