The diencephalon, as all other subdivisions of the nervous system, is arbitrarily delimited, and contains within its limits a series of centers associated with various functions; these centers are not only interrelated with each other, but they are as intimately related with lower and higher segments of the nervous system. Thus the degree of differentiation of the diencephalon in any form is in direct relation with the extent of development of the peripheral nervous system and the degree of differentiation of the spinal, bulbar and pontile centers. Moreover, the possible complexity of the striatal and cortical centers is limited and determined by the complexity of the diencephalon.
A study of the phylogenetic development of the nervous system reveals a gradual increase in the longitudinal conduction pathways and a progressive differentiation through the whole extent of the nervous system, which at certain stages may involve more particularly a single segment, but
HUBER GC, CROSBY EC. SOMATIC AND VISCERAL CONNECTIONS OF THE DIENCEPHALON. Arch NeurPsych. 1929;22(2):187-229. doi:10.1001/archneurpsyc.1929.02220020003001