Functional factors in the morpliolof^y of thc forcbrain of fishes 175 



cephalon by olfacto-habenular and olfacto-hypothalamic tracts of the 

 tliird order. 



All parts of the telencephalon which receive fibers of the tractus ol- 

 factorius comprise the área oHactoria. In all groups ot fishes which have 

 been adequately described the entire telencephalon except the small área 

 somática (and in some species perhaps the nucleus preopticus) is reached 

 bv axons of mitral cells of the bulbus olfactorius, that is, it is área olfac- 

 toria. And between the non-olfactory área somática and the área olfac- 

 toria there is a transitional región which is bridged by olfacto-somatic 

 correlation fibers, so that the área somática is not entirely free from ol- 

 factory influence. There are ascending and descending fibers connecting 

 the área somática with the adjacent somatic thalamus. But by far the 

 most important ascending systems which enter the telencephalon come 

 from the hypothalamus. 



In the lowest vertebrates, where the telencephalon is broadly connec- 

 ted with the diencephalon, the fibers connecting these regions are dif- 

 fusely arranged. In other cases, where the di-telencephalic boundary is 

 marked by a deep cleft dorsally at the level of attachment of the velum 

 transversum, the longitudinal tracts are crowded ventrally in the narrow 

 caudal part of the telencephalon médium which was formerly called pre- 

 thalamus. In this contracted space the habenular fibers accumulate along 

 the dorsal border, where they form the stria medullaris. The other longi- 

 tudinal fibers form dense fasciculi which in the aggregate are called the 

 basal forebrain bundles. 



The basal forebrain bundles in all cases tend to divide into a medial 

 and a lateral series. The medial bundle ahvays connects the área oltacto- 

 ria medialis, nucleus preopticus, and hypothalamus, so that these ventro- 

 medial centers are closely knit together and can act concurrently. (3ther 

 fiber tracts may be added to this primary system in various patterns de- 

 pending on the topographic relations oí the parts. 



The lateral forebrain bundle always connects the topographically la- 

 teral parts of the telencephalon with the hypothalamus, ventral part ot 

 the thalamus, and pedunculus cerebri. To these systems there may be 

 added in various fishes fibers from other parts of the telencephalon. 1 he 

 connections of the área olfacto-somatica are typically made through the 

 lateral bundle. The connections between the área somática and the tha- 

 lamus in more generali/ed fishes run separately from other systems of 



