276 NERVOUS SYSTEM OF VERTEBRATES. 



of fishes and other vertebrates. They are tertiary olfactory fibers 

 and it is evident that the whole hypothalamus receives such fibers 

 in all vertebrates, although the caudal part, the corpora mam- 

 millaria, is especially developed as the end-nucleus of olfactory 

 fibers of the fourth order, the fornix. Considering the conditions 

 in fishes it is to be expected that a tertiary gustatory tract in 

 mammals will be found ending in the tuber cinereum. 



In fishes a tract from the tectum mesencephali enters the hypo- 

 thalamus, the tractus tecto-lobaris. This tract would provide 



Fig. 138. — Sagittal section of the tuber cinereum of the rat of eight days. From 

 Cajal (Textura, etc.). A, anterior or chief nucleus; B, posterior or accessory nucleus; 

 C, internal nucleus of corpus mammillare; D, nerve tract coming from the septum 

 pellucidum (tractus olfacto-hypothalamicus medialis); E, efferent fibers from the 

 tuber which disappear in the central gray matter; F, efferent tract from the corpus 

 mammillare; G, upper nucleus of the tuber; K, optic chiasma. 



for the correlation of olfactory and visual or cutaneous impulses. 

 The homologue of this tract in mammals has not been recognized, 

 but it is known that fibers from other sources besides the olfactory 

 systems described above end in the tuber cinereum. 



The tract going out from the corpora mammillaria, as already 

 mentioned, is the same in all vertebrates. Tracts going from the 



