THE COMMISSURES 295 



COMMISSURA POSTOPTICA 



The complex com. postoptica is represented in mammals by the 

 supra-optic commissures, but its composition is so different in 

 urodeles and mammals that exact homologies cannot be established. 

 Further analysis of intervening species is requisite before these rela- 

 tionships can be clarified. 



In Amblystoma this complex includes decussating fibers derived 

 from the superior and inferior colliculi, the entire diencephalon, the 

 amygdala, and the subpallial olfactory field of the cerebral hemi- 

 sphere. Many of these tracts have collateral connections along their 

 courses, both before and after crossing, and are accompanied by 

 uncrossed fibers. There are also strictly commissural fibers connect- 

 ing some of these regions (for evidence of such fibers in the frog see 

 '25, p. 480). These decussating systems connect, after crossing, with 

 extensive fields of the intermediate and motor zones, including the 

 strio-amygdaloid area, preoptic nucleus, hypothalamus, ventral thal- 

 amus, geniculate neuropil, dorsal tegmentum, peduncle, isthmic teg- 

 mentum, and bulbar tegmentum. The posterior part of the postoptic 

 commissure is comparable with the com. tuberis of some other verte- 

 brates and consists mainly of decussating fibers from the ventral part 

 of the hypothalamus to the peduncle and interpeduncular nucleus. 

 The direction of conduction of most of these fibers has not been 

 clearly determined, though most of the larger systems evidently 

 converge from the sensory zone into the motor field. 



The fibers of few of these components are assembled in well- 

 organized tracts ; most of them are so dispersed and commingled that 

 analysis is very difficult. Some of them are myelinated, and these 

 tend to be assembled in recognizable tracts. These myelinated tracts, 

 as seen in Weigert sections, were the first to be described, but their 

 distribution after crossing baffled analysis. These tracts are accom- 

 panied by much larger numbers of unmyelinated fibers in more dis- 

 persed arrangement. The courses of some of these have been revealed 

 by elective Golgi impregnations, and other systems have been clari- 

 fied by study of the sequence of their development as published in a 

 series of papers from 1937 to 1941. In view of the complexity of these 

 connections and the technical difficulties encountered in their study, 

 it is not surprising that the earlier descriptions were incomplete and 

 not free from error. It is believed that now it is possible to present an 

 analysis of this complex which, though still incomplete, reveals its 

 major features. 



