THE SPINAL CORD AS A CONDUCTING PATH 



623 



The Distribution of the Impulses Derived from the Posterior 

 Roots. — The posterior roots of the cord are very important "feeders" 

 of the central nervous sytem. Together with the afferent fibers of 

 the cranial nerves, they constitute the only means by which the higher 

 centers may be influenced by impulses generated peripherally. These 

 impulses embrace first of all the superficial and deep sensations of 

 touch, pain and temperature, as well as those derived from the re- 

 ceptors in the muscles and tendons, having to do with the muscle- 

 sense. On their arrival in the terminals of the posterior root fibers, 



BotMlaleral impulsRS WnSerlyiru) muscular eensibililij—i.e. seme of passive posilion and of 

 movement, also nf touch and pressure for a few segments. 



I, 



Z., 



1. Helerolaieral 



uriconscioua afferent 

 impulses underlying 

 I muse, co-ordination 

 and reflex tone. 



8. All impulses 

 of pain, of heal, 

 and of cold 

 {Helerolaieral). 



9. Heterolateral impvleea of 

 touch and presswe,. 



Fig. 310. — Diagbam to Illustrate the Termination op Peripheral Afferent 

 Fibers in the Spinal Cord, and the Origin of the Secondary Central Paths, with a 

 Brief Summary of Their Function. 



1, Bundles of fibers passing up in the posterior column — many myelopetal (to sp. 

 cord) and the remainder bulbopetal (to p. col. nuclei) ; 2, fibers terminating around the 

 cells of Clarke's column; 3, fibers arborizing around cells in the posterior horn, and inter- 

 mediate gray matter; 4, ditto around the anterior horn-cells; 5, ditto swerving into the 

 lateral column to neighboring gray matter; 6, direct, or dorsal spinocerebellar tract; 

 7 and 8, Gowers' tract, i.e. (7) ventral spinocerebellar tract; (8) spinothalamic and 

 tectal tracts; 9, ascendingtract in the anterior column. {Starling after W. Page May.) 



they are distributed to those particular groups of cells and fibers 

 which are directly concerned with their conduction to other parts. 

 Their distribution is effected as follows: 



(A) Impulses Retained at the Level of their Entrance into the Cord. — They are 

 reflex in their nature and gain the corresponding effector by way of the anterior 

 root fibers. This transfer of the afferent impulses into efferent ones is accomplished 

 directly through the intervention of the cells of the anterior and lateral horns of the 

 neighboring gray matter. While the largest number of these impulses remain 

 confined to the same side of the cord, some also seek the opposite spinal gray matter 

 and opposite anterior root by way of the posterior white commissure. 



{B) Impulses Seeking Levels above and below their Level of Entrance. — They are 

 distributed to: 



