Functional factors in thc morpholoí^'V of the forebr.-iin of fishes 179 



ancl from them fibers have grown foruard effecLing functional correlation 

 with the olfactory centers. In fishes the highest optic, acoustico-lateral 

 and cutaneous correlation centers are in the roof of the midhrain. The 

 thalamic centers of these somatic sensory systems are relatively small and 

 do not assume great importance in any animáis of lower rank than the 

 Anura. The hypothalamic centers, however, are large and highly special- 

 ized in all ot these lower iorms. Diversity in the physiological charac- 

 teristics of the ascending non-olfactory systems is the chief factor which 

 has shaped the course of differentiation within the olfactory field. 



The ascending visceral and gustatory fibers enter the hypothalamus, 

 and to these are added important tecto-hypothalamic connections from 

 the roof of the midbrain. Here converge olfactory, gustatory, visceral 

 and somatic systems of considerable complexity, and this seems to be the 

 most important center of correlation for these systems in the brains 

 of fishes. 



The epithalamus of lower vertebrates is known to be connected by 

 correlation tracts with the roof of the midbrain and the dorsal part of the 

 thalamus and primitively served as an important center of adjustment 

 between these strictly somatic sensory centers and the olfactory centers, 

 with perhaps important epiphyseal connections. In higher vertebrates 

 the afterent non-olfactory fibers entering the habenula seem to become 

 relatively less and the organ functions chiefly as a station in the efferent 

 discharge pathway from the olfactory centers of the telencephalon, the 

 olfacto-somatic correlation mechanisms having been transferred inte tel- 

 encephalic territory. 



The thalamus proper, as we have seen, attains no very great develop- 

 ment in any fishes. Much of its tissue is in early embryonic stages eva- 

 ginated to form the retinas and the residue is a somatic sensori-motor 

 correlation center, receiving fibers from the optic tracts and the roof of 

 the midbrain and discharging fibers into the pedunculus cerebri. The 

 details of its internal structure in fishes are imperfectly known. The lim- 

 its between thalamus and epithalamus above and thalamus and hypo- 

 thalamus below seem not very well defined and it is probable that in 

 these forms the thalamus is not so clearly differentiated into pars dorsa- 

 lis thalami (sensory) and pars ventralis thalami (motor) as is the case in 

 the Amphibia. In some fishes numerous fibers from the área olfactoria 

 seem to pass directly into the thalamus. In higher vertebrates, too, some 



