DiEXCEPHALOX 75 



terior extremity, then, turning sharply ventrad, they run 

 straight to the interpeduncuhir nucleus as the fasciculus 

 retroflexus (Pis. XI\'.-X\'III.). Thus the habenular nuclei 

 are a relay station on the descending olfactory pathway and 

 probably serve for the correlation of the olfactory impulses 

 with somatic sensory ones received from neighbouring parts 

 of the thalamus (Herrick). This whole system, as already 

 pointed out, is considerably larger in macrosmatic animals, 

 such as the rat, than in microsmatic forms, such as man. 



The hypothalamus forms the floor of the third ventricle, 

 and comprises the pars optica hypothalami, the tuber cinereum 

 and the infundibultcm , and the mamillary bodies. The pars 

 optica hypothalami is part of the telencephalon and will 

 therefore be considered later. 



The mamillary bodies are a pair of rounded eminences 

 at the posterior extremity of the ventral surface of the dience- 

 phalon in the human subject. In the rat, however, as in 

 many other lower mammals, the two bodies are fused into 

 a single median mass (Pis. II., XV., XXVL). This fusion 

 is not only external but also internal, the bodies being separ- 

 ated in man by a ventral extension of the third ventricle 

 which does not occur in rodents. 



Superficially, the bodies are covered by a very thin capsule 

 of white fibres. Each mamillary body consists of two main 

 nuclei, a larger medial one and a smaller lateral one, and 

 between these penetrates a great mass of descending olfactory 

 fibres of the fourth order — the column of the fornix {columna 

 fornicis) (Pis. X\\-XXII.). Many of these fibres (or at least 

 many collaterals from them) end in the mamillary nuclei, 

 dithers, however, pass dorso-medially, becoming more scat- 

 tered as they do so, and form a diffuse decussation {the siipra- 

 mamitlary decussation) (PI. X\\), after which they descend 

 through the tegmentum. 



Another tract belonging to the mamillary body which is 

 well developed in the rat, and which is probably afferent, is 



