Houser, Zhe Neurones of a Selachian. I4I 
with the growing importance of the interbrain as a primary 
optic centre, however, there has been a related decline of the 
midbrain roof. . And hence it is that the optic lobes of the fish 
appear so disproportionately large in comparison with the 
homologous parts of the mammalian brain. 
2. Lpithalamus: The Nuclet Habenulae. 
The nuclei habenulae, or ganglia habenulae of authors, 
rise considerably above the level of the thalami (Fig. 1, x. %.), 
the pair meeting each other to forma conspicuously arched 
bridge across the third ventricle at its posterior end. The 
epiphysis springs from the middle of the arch. The left nucleus 
is a little larger than the right one, its margin extending slightly 
more anteriorly. 
The nucleus habenulae is an important relay-centre, and so 
its structure exhibits many nerve-fibres taking various direc- 
tions, between which there are neurones and supporting ele- 
ments. The tractus olfacto-habenularis (Section VIII, 1, c) ter- 
minates here, and the neurones of the nucleus give origin to 
the tractus habenulo-peduncularis, (the bundle of MEynert, and 
the fasciculus retroreflexus, of authors). 
A representative neurone is shown in Fig. 25. The size of 
the entire element is rather large. The cell-body tends to retain 
a rounded form, although diverted from this condition more or 
less by the thickened bases of the dendrites. The dendrites 
are some three or four in number, gnarled and irregular pro- 
cesses, branching only a few times, and extending far outward 
in every direction from their points of origin. The surface of 
a dendrite is roughened by nodal thickenings, knobs, and a few 
gemmules. The axone arises directly from the cell-body in 
all of the instances observed. Its course is traceable for only 
a short distance in a transverse section, since it soon turns pos- 
teriorly into the tractus habenulo-peduncularis. This impor- 
tant tract takes the usual course toward the base of the mid- 
brain. Its termination occurs there in the nucleus interpedun- 
cularis. The significance of the tract is to be interpreted as a 
part of the olfacto-motor complex, discussed more particularly 
in Section VIII. 
