468 C. JUDSON HERRICK 



In cyclostomes another and smaller part of the secondary- 

 olfactory center remains permanently in the dorsal wall of the 

 telencephalon medium. There is here a dorso-median ridge 

 termed by Johnston ('02) the epistriatum and by other authors 

 the dorsal part of the praethalamus, the rostral end of which is 

 apparently telencephalic. That is, the evagination of the cere- 

 bral hemisphere at first, like that of the optic vesicle, involves 

 neither the extreme ventral nor the extreme dorsal part of the 

 lateral wall of its segment. Fig. 72 is a picture of the probable 

 relations in a hypothetical vertebrate ancestor, in which neither 

 the optic vesicles nor the cerebral hemispheres have evaginated 

 from the neural tube. The tissue which gives rise to the optic 

 vesicle and retina in true vertebrates is indicated at R. Its 

 dorsal position is based on Johnston's discussion ('09, p. 479). 

 At is shown the tissue which evaginates in vertebrates to form 

 the olfactory bulb. It is surrounded by secondary olfactory tis- 

 sue mere or less of which is evaginated into the cerebral hemi- 

 sphere in different vertebrates. The dorso-median di-telenceph- 

 alic ridge (d.m.r.) is a part of this secondary olfactory center, as 

 is also the ventro-median olfactory nucleus (n.olf.v.m.). All 

 parts of the secondary olfactory nucleus are connected by fiber 

 tracts with both the hypothalamus and the epithalamus. 



The recurving of the dorsal lamina around the rostral end of the 

 ventral lamina brings a part of the secondary olfactory area into 

 immediate contact with the hypothalamus, which as pointed out 

 above (p. 466), is occupied chiefly by visceral motor correlation 

 tissue. The hypothalamus, accordingly, becomes the great 

 avenue of discharge for olfactory reflexes of the visceral (intero- 

 ceptive) systems and the center for their coordination, a relation 

 which is preserved with great constancy throughout the whole 

 series of vertebrates. 



The epithalan ic connection of the secondary olfactory area has 

 a quite different significance. I have elsewhere ('08) commented 

 on the fact that the sense of smell, unlike the chemical senses in 

 general, functions both as an interoceptor and as a distance re- 

 ceptor. It is the latter function which requires intimate rela- 

 tions with the exteroceptive centers of the dorsal part of the thai- 



