THE IL\BENULA AND ITS CONNECTIONS 253 



ponent of an equilibrated dynamic system, of which the basal fore- 

 brain bundles comprise the activator component. On this hypothesis 

 the activation of all olfacto-motor systems goes out through the basal 

 forebrain bundles, and this activation is accompanied by an inhibi- 

 tion of all conflicting activities, the inhibitory component of the re- 

 action being centered in the interpeduncular nucleus. This implies 

 that inhibitory influences derived from exteroceptive fields are trans- 

 mitted to the interpeduncular nucleus by the f . retroflexus and from 

 interoceptive fields by the mamillo-interpeduncular tract. This hy- 

 pothesis seems to be consistent with the known structure, but it lacks 

 experimental proof. If there is factual basis for it, the activating and 

 inhibitory systems must not be regarded as independent units of 

 structure; they are everywhere interconnected, and all their activi- 

 ties are balanced one against the other in an integral dynamic system. 

 The literature contains many fragmentary accounts of the habenu- 

 lar connections of Amphibia, with some conflict of observation and 

 confusion of nomenclature. When these descriptions are assembled, 

 together with additional observations here reported, the salient fea- 

 tures of the habenular connections may be grouped into three classes. 

 These are: 



1. There are two groups of commissural connections between the 

 two cerebral hemispheres. Group 1 comprises the commissura su- 

 perior telencephali, with fibers from the anterior olfactory nucleus, 

 piriform area, and amygdala (components 3, 4, 5, and 6 of the list 

 given on pp. 257-60). This is the larger part of the habenular com- 

 missure. It was seen by van Gehuchten in Salamandra ('97rt), though 

 its true origins were not recognized and his belief that none of these 

 fibers have connection with the habenular nuclei needs confirmation. 

 Group 2 is the com. pallii posterior (component 8 of the list). These 

 fibers come from the primordium hippocampi and are probably com- 

 parable with the com. aberrans of some reptiles. 



2. The habenula is an important way-station for through traflSc 

 from all parts of the cerebral hemisphere and preoptic nucleus to the 

 brain stem below this level, particularly to the peduncle and inter- 

 peduncular nucleus. All these conduction pathways except the com- 

 missural fibers are interrupted by synaptic junctions in the habenula, 

 and the habenular synaptic field is connected with neighboring re- 

 gions by fibers passing in both directions. 



3. The through traffic is influenced by these local connections with 

 areas of the sensory zone concerned primarily with exteroceptive 



