CHAPTER XVII 

 DIENCEPHALON 



GENERAT. FEATURES 



IN THE f()ll()win,t>- analysis tlie structures at the di-telenceplia ic 

 junction are omitted, since they are fully described in the next 

 chapter. The retina and the optic nerve are integral parts of the 

 diencephalon. The retina itself is more than a simple receptor, for it 

 contains elaborate apparatus for analysis and synthesis of v'sual 

 excitations (Polyak, '41). 



The diencephalon combines characteristics of the sensory, inter- 

 mediate, and motor zones, and the tissues involved are so closely 

 interwoven that toi)ographic subdivisions into zones is arbitrary and 

 misleading. The lai-gest number of afl'erent fibers enter by the optic 

 nerve and are distributed to the thalamus, pretectal nucleus, and 

 hypothalanms, in addition to their mesencephalic connections. The 

 nervus terminalis has widely spread terminals in the preoptic nucleus 

 and the hypothalamus. The parietal nerve contains a few fibers of 

 uncertain connections and functions. The hypo])hysial nerve carries 

 many fibers to the ])ars nervosa of the hypophysis ami a smaller num- 

 ber to the j)ars distalis. 



Between the central stations of these peripheral nerves and inter- 

 penetrating them is correlating tissue of intermediate-zone type. As 

 repeatedly emphasized in several preceding contexts, the diencepha- 

 lon as a whole (except for the retina) is a transitional sector of the 

 brain, inter})olated between the olfactory field anteriorly and all 

 other sensori-motor fields posteriorly. This apparatus of correlation 

 is supplementary to the more direct paths of through traffic which 

 organize the basic patterns of the stable action system characteristic 

 of the species. In urodeles the most direct descending olfactory con- 

 nections are by way of the tractus olfacto-peduncularis for somatic 

 responses and to the hy})othalamus for visceral responses. Additional 

 diencephalic olfactory connections are ancillary to these. The ap- 

 paratus of other stereotyped patterns of sensori-motor adjustment is 

 similarly organized in the stem below the diencephalic level. These 



