REGIONAL ANALYSIS 51 



24). This is an undifferentiated primordium of the basal optic nucleus 

 and some other structures of the mammalian brain (pp. 35, 221). 

 It is related with the olfacto-visceral functions of the hypothalamus 

 and probably also with conditioning of the fundamental peduncular 

 activities. 



10. VENTRAL THALAMUS 



There are anterior and posterior sectors of the ventral thalamus 

 which differ in embryological origin (p. 239) and in certain connec- 

 tions of intermediate-zone type. Both sectors are here included in the 

 motor zone because their chief efferent connections resemble those of 

 the "peduncle," of which the posterior part is physiologically an 

 anterior extension. The ventral thalamus is the primordium of the 

 motor field of the mammalian subthalamus. The anterior sector con- 

 tains a nucleus specifically related to the stria medullaris and 

 amygdala and, above this, the eminentia thalami, which is a bed- 

 nucleus of tracts related to the primordium hippocampi (chap, xviii; 

 figs. 16, 17, 19, 20, 96). 



The ventral thalamus and peduncle of urodeles form a single mas- 

 sive column, which is anatomically well defined. The specialized 

 structures derived from it in mammals are dispersed among large 

 masses of tissue of more recent phylogenetic origin; but in the human 

 brain this region still retains cerebral control of the primordial co- 

 ordinated movements of the musculature of the eyeballs and of the 

 trunk and limbs. 



11. THE RETINA AND ITS CONNECTIONS 



In early embryonic stages the retina is part of the brain, and, as 

 development advances, it absorbs much of the diencephalic sector of 

 the early neural tube. This precociously accelerated development re- 

 sults from the dominance of vision in exteroceptive adjustment from 

 the time that the larva begins to feed. For further details of this 

 development and of the organization of the visual-motor apparatus 

 see chapter xvi. 



1-2. IIABEN(jL.\ 



As described in chapter xviii, this specialized part of the epithala- 

 mus receives fibers from almost all parts of the telencephalon and 

 diencephalon and from the tectum (fig. 20). The habenular commis- 

 sure connects the two habenulae, and it also contains two commis- 

 sures of pallial parts of the hemispheres — commissura pallii posterior 

 and com. superior telencephali. The chief efferent path from the 



