CORRELATING CENTERS IN THE DIENCEPHALON. 289 



In the roof of the diencephalon two decussations are present, 

 the well-known superior orhabenular commissure and a decussation 

 closely related to the base of the epiphysis known only in a few 

 forms. The habenular commissure contains decussating fibers 

 from the olfactory nuclei of the forebrain (tractus olfacto-habenu- 

 laris) and also probably true commissural fibers. It is to be 

 compared with the inferior cerebellar commissure. Decussating 

 fibers of the second and third order are present in each case and 

 the nuclei in both cases are specialized parts of the substantia 

 reticularis related to visceral functions. 



The post-epiphysial decussation has been described only in 

 ganoids (Johnston), bony fishes (Holt) and the horse (Favoro). 

 It is poorly understood but deserves further study. 



The anterior commissure, which will be described in the next 

 chapter, is a dorsal decussation which, so far as at present known, 

 is related in lower vertebrates to centers belonging to the visceral 

 sensory system (olfactory and gustatory). In higher vertebrates 

 two large commissures, one related to the olfactory cortex, the other 

 to the somatic pallium, are developed from the anterior commissure. 



The dorsal decussations may be summarized by saying that 

 the mixed dorsal decussation of the cord has been differentiated 

 in the brain into separate somatic and visceral sensory decussations. 

 The commissura infima, the inferior commissure of the cerebellum, 

 the habenular commissure and the anterior commissure represent 

 the visceral portion. The superior commissure of the cerebellum, 

 the dorsal decussation and posterior commissure in the mesen- 

 cephalon (and the corpus callosum in the telencephalon) represent 

 the somatic portion. 



The ventral decussation of the spinal cord consists of internal 

 arcuate fibers and of the neurites of heterolateral tract cells. It is 

 in smaller part a decussation of secondary sensory tracts, in larger 

 part belongs to the substantia reticularis. In the medulla 

 oblongata the secondary sensory elements (lemniscus system) are 

 more numerous and in the base of the mesencephalon a large 

 number of fiber tracts decussate in the region of the nuclei of the 

 III and IV nerves. These tracts are in part tertiary sensory 

 tracts, in part descending tracts from somatic and visceral correlat- 

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