CORRELATING CENTERS IN THE DIENCEPHALON. 287 



centers lying in the two interbrain segments in such a way that 

 a part of this column lies far dorsad (nucleus habenulae) and a 

 part far ventrad (hypothalamus). This has been fully set forth 

 above. 



Finally, a considerable part of the adult midbrain and interbrain 

 consists of substantia reticularis or nuclei derived from it, whose 

 morphological and genetic relations to the primary functional 

 divisions are unknown. 



The commissures of the brain. — ^The facts regarding the 

 cliief brain commissures which have been scattered through the 

 foregoing pages may be brought together here for convenience 

 of reference. It should be noted at the start that the fiber- 

 crossings in the lower vertebrates are for the most part mere 

 decussations. The dorsal decussation of the spinal cord in higher 

 vertebrates contains visceral sensory (sympathetic) fibers, collat- 

 erals from cutaneous and visceral fibers, and secondary fibers 

 from both somatic and visceral sensory columns. At the junc- 

 tion of the spinal cord and brain, i.e. just behind the choroid 

 plexus of the ventricle, in all vertebrates this decussation is greatly 

 enlarged. This enlarged portion, known as the commissura infima 

 (Figs. 81, 82, 83, 90, 92), is due chiefly to an increase of the vis- 

 ceral sensory fibers from the roots of the VII, IX and X nerves 

 and of secondary visceral fibers arising from the nuclei of those 

 nerves. Other fibers in this commissure come from the cells of the 

 nucleus funiculi. The dorsal decussation of the cord is therefore 

 mixed somatic and visceral in character. These two components 

 must be rigidly distinguished if the dorsal decussations of the brain 

 are to be understood. 



The dorsal decussation of the medulla oblongata is not obliter- 

 ated on account of the non-nervous roof, but its elements are 

 crowded forward or backward. Behind the choroid plexus the 

 commissura infima contains the visceral sensory elements proper 

 to the segments of the VII, IX and X nerves. It is probable that 

 the course of the root fibers of these nerves within the brain has 

 been influenced by the crowding backward of their decussation 

 and median nucleus by the choroid plexus. It is further prob- 

 able that those fibers which take this caudal course are the more 



