176 C. Judson Herrick 



fibers, but in teleosts they join the lateral forebrain bundle, as they do in 

 higher brains. 



The composition of the basal forebrain bundles is so diverse in the 

 various groups of fishes, depending on the configuration of the telence- 

 phalon, that no general statement can be framed which will apply to all 

 of the cases; but there is a definite tendency toward the segregation of 

 fibers related to the habenula in the stria meduUaris, of fibers connected 

 with the área olfactoria medialis and the (topographically) medial part of 

 the área olfactoria dorsalis in the medial bundle, and of all other longitu- 

 dinal tracts in the lateral bundle. This tendency reaches its consumma- 

 tion in the Amphibia, where the entire telencephalon except the nucleus 

 preopticus is evaginated to form a hoUow cerebral hemisphere. Here 

 the medial forebrain bundle, or fasciculus medialis telencephali, is defini- 

 tely related with the medial wall of the hemispheric vesicle (área olfacto- 

 ria medialis including the septal complex, and primordium hippocampi) 

 and the lateral bundle, or fasciculus lateralis telencephali, is similary rela- 

 ted with the lateral wall (área olfactoria laterahs, lobus pyriformis, pri- 

 mordial striatum and amygdalaj. In still higher forms projection fibers 

 of the general cortex are added to the lateral bundle. 



In urodele Amphibia the lateral wall of the cerebral hemisphere is 

 everywhere reached by fibers of the tractus olfactorius, except perhaps a 

 small región at the caudal end of the ventro-lateral part. All (or nearly 

 all) of this lateral wall, therefore, belongs in the área olfactoria. The hy- 

 pothalamus is divided by a sharp sulcus hypothalamicus and by internal 

 structure into two parts — pars ventralis and pars dorsalis. The thala- 

 mus proper is also divided into pars dorsalis and pars ventralis by a sul- 

 cus medialis thalami, the pars dorsalis being a somatic sensory correla- 

 tion center (optic, acoustic, somesthetic) and the pars ventralis being a 

 center of motor discharge into the pedunculus cerebri. For the details 

 of these connections see my papers published in 1917 and 1921. 



The medial wall of the urodele hemisphere is connected by ascending 

 and descending fibers of the medial forebrain bundle with the nucleus 

 preopticus and the pars ventralis hypothaiami. The lateral wall is simi- 

 larly connected through the ascending and descending fibers of the ol- 

 factory projection tract of Ramón y Cajal with the pars ventralis hypotha- 

 iami. These fibers run in the lateral forebrain bundle. The olfactory 

 projection tract is a modified derivative of the tractus pallii of fishes and 



