178 C. Judson Herrick 



In siimmary , there are in fishes four chiefsystems of longitudinal con- 

 duction paths: (l) The stria medullaris system discharging into the ha- 

 benula. (2) The medial forebrain bundle connecting the área olfactoria 

 medialis, nucleus preopticus, and hypothalamus. In elasmobranchs it 

 receives fibers from the medial (inverted) part of the área olfactoria dor- 

 salis which Johnston compares with the columna fornicis of higher brains. 

 In teleosts it receives fibers from all parts of the área olfactoria dorsalis 

 (Holmgren). (3) The tractus pallii complex connecting the área olfacto- 

 ria dorsalis and the hypothalamus and probably represented in higher 

 brains by the olfactory projection tract. W^ith this may be associated 

 the descending fibers from the área olfactoria lateralis to the hypothala- 

 mus, or tractus olfacto-hypothalamicus lateralis. (4) Ascending and des- 

 cending somatic projection fibers passing between the área somática and 

 the thalamus. Ascending fibers of this system (tractus thalamo-fronta- 

 lisj may ascend farther forward into the área olfacto-somatica (which is 

 differentiated within the área olfactoria lateralisj either directly (Acipen- 

 ser, Johnston, teleosts, Sheldon) or after synapse in the área somática 

 (elasmobranchs, Johnston). Fibers of this system which descend from 

 the área somática are here termed, following Johnston, somatic projection 

 tract. Those which descend from the área olfacto-somatica farther for- 

 ward are termed tractus strio-thalamicus, though it should be borne in 

 mind that this área is a mixed somatic and olfactory center and, accor- 

 dingly, the tractus strio-thalamicus of fishes .functions also as a tractus 

 olfacto-thalamicus and hypothalamicus lateralis. In higher vertebrates the 

 olfactory component of this complex becomes progressively less and the 

 somatic component greater. This fourth system is the precursor of 

 the thalamo-striatal, strio-thalamic and infernal capsule systems of higher 

 orains. The third and fourth of these systems are represented in the la- 

 teral forebrain bundle of Amphibia. 



The subdivisions of the forebrain. 



Whatever mav have been the most primitive organization of the tel- 

 encephalon, it is clear that in all living fishes this entire región and a very 

 considerable portion of the diencephalon are dominated by the olfactory 

 system. The primary non-olfactory centers lie farther back (spinalward). 



