CHAPTER XXI 

 THE COMMISSURES 



GENERAL CONSIDERATIONS 



THE commissures are in two series, dorsally and ventrally of the 

 ventricles. The fibers which cross the mid-plane are of two sorts: 

 (1) some are strictly commissural, connecting corresponding regions 

 of the two sides; (1) most of them are decussations, like the optic 

 chiasma, connecting dissimilar regions. Those of the dorsal series 

 include fibers of correlation which arise and terminate within the 

 sensory zone and also connections between the sensory zone and 

 other zones. Those of the ventral series are concerned in the main 

 with motor co-ordination on the two sides of the bod3% some passing 

 from sensory and intermediate zones to the motor zone, others lying 

 wholly within the motor zone, and both sorts being accompanied by 

 uncrossed fibers. 



In addition to the crossed systems of correlation and co-ordination 

 to which reference has just been made, it is a noteworthy fact that 

 the main lines of fore-and-aft ascending and descending conduction 

 in the brains of all vertebrates decussate, so that the adjusting cen- 

 ters for organs on the right side of the body are in the left side of the 

 brain and vice versa. This applies especially to the apparatus of 

 somatic adjustment, but not so generally to the visceral systems. 

 Most of the fibers of the ascending secondary visceral-gustatory tract 

 {tr.v.a.) and of the olfacto-hypothalamic tracts are uncrossed. The 

 proprioceptive systems are both crossed and uncrossed. 



The reason for the decussation of the major conduction pathways 

 of the somatic sensori-motor systems has puzzled neurologists for 

 many years, and fantastic theories have been expressed, some bear- 

 ing the names of great masters — Wundt, Flechsig, Cajal, and others. 

 This extensive literature has been reviewed by Jacobsohn-Lask ('24), 

 who, instead of elaborating a theory in terms of the highly specialized 

 human brain, like his predecessors, reviews the entire history of the 

 evolution of the nervous system, from its first appearance in coelen- 

 terates, in relation to the bilateral symmetry of the body. None of 

 these speculations have yielded satisfying conclusions. 



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