302 THE BRAIN OF THE TIGER SALAMANDER 



Postoptic neuropil. — In the mid-plane, the postoptic decussations 

 occupy most of the chiasma ridge, and these fibers are enveloped on 

 all sides except ventrally by a gray layer, the bed-nucleus of the 

 postoptic commissure. This nucleus is expanded laterally. From this 

 gray layer, richly arborized ependymal elements and dendrites of 

 neurons are spread among the decussating fibers. Similar long den- 

 drites enter it from all surrounding parts, including the nucleus 

 magnocellularis, from which tr. hypophysius arises ('42, fig. 51). 

 The entire chiasma ridge is also permeated with dense axonic neu- 

 ropil which is continuous with that of surrounding parts. This neu- 

 ropil receives terminals and collaterals of axons of hypothalamic 

 neurons, medial forebrain bundles, tractus preopticus, and most of 

 the decussating systems. The chief outflow from it seems to be by 

 axons of the nucleus of the postoptic commissure directed into tr. 

 hypothalamo-peduncularis. Other fibers enter the medial forebrain 

 bundle (for further details see '42, p. 219; Necturus, '336, p. 251; 

 'Mb, p. 383). 



This neuropil is clearly one of the major adjusting centers of the 

 urodele brain. Situated in the center of the great olfacto-visceral field, 

 its connections indicate that it may be activated from every correla- 

 tion center of the cerebrum. Undoubtedly it plays an important part 

 in all general visceral activities. It is equally evident that there is no 

 provision here for localization of specific functions. This is probably 

 the undifferentiated primordium from which some of the specialized 

 hypothalamic nuclei of mammals have been elaborated. 



COMMISSURA TUBERCULI POSTERIORIS 



The commissure of the tuberculum posterius was defined in 1917 

 (]). 224) as the ventral mesencephalic decussations between the 

 infundibulum and the fovea isthmi. Posteriorly of the latter the 

 ventral tegmental commissure extends backward without interrup- 

 tion through the rhombencephalon and spinal cord. In Necturus 

 there is a very short interruption of the ventral commissural system 

 at the fovea isthmi ('30, p. 89), but in most urodeles this gap does not 

 appear. 



The composition and arrangement of these ventral commissures 

 are very diversified in different vertebrates. The hypothalamic con- 

 nections at the anteroventral end of the commissure of the tubercu- 

 lum posterius of Amblystoma are in some other vertebrates widely 

 separated as the retro-infundibular decussations, or decussatio hy- 



