Houser, Zhe Neurones of a Selachian. 143 
from the base of the cell, and the axone passes ventrally, branch- 
ing profusely (Fig. 27). A neurone from the side-wall (Fig. 
28) invariably has its axone emerging from the side of the cell, 
taking a course toward the limitans externa for a short distance, 
then branching in a T-shaped manner. The fibres thus formed 
run parallel with the surface of the brain, one turning into the 
ventral part of the lobus, the other pursuing an arcuate course 
out of the hypothalamus (Fig. 27, f. 0.). Such a fibre is 
marked by varicosities at intervals, and it bears collateral 
branches (Fig. 28). 
The internal structure of two neurones from the lobus 
inferior is given in Fig. 64. The figure also shows how closely 
these neurones are packed. The nucleus is only fairly large, 
and it is surrounded by a thick layer of cytoplasm on all sides. 
The chromatic substance is scanty in amount; it is distributed 
‘ina few thin strands. The tigroid masses are not numerous, 
and most of them are quite small, with just a few large ones 
distributed at irregular intervals in the peripheral region of 
the cell. 
4. Supporting Elements. 
Associated with the many and crowded nerve-tracts charac- 
terizing the structure of the interbrain, there is to be noted a 
corresponding development of both neuroglia and ependyma. 
A neurogliar element has a few stout processes radiating 
in every direction from the cell-body, often pursuing a markedly 
tortuous course. They ramify in an exceedingly complicated 
manner, the finest twigs interlacing to forma dense mat; see 
Fig. 29. Neuroglia is found in the nuclei of the interbrain, 
serving to support both the neurones and the terminal fibres 
occurring there. 
Ependyma is found in all parts of the interbrain. The 
ependymal fibre always extends throughout the entire thickness 
of nervous matter, from the ventricle to the limitans interna. 
The fibre branches midway in its course, the several limbs often 
diverging considerably. The entire fibre-system bears a multi- 
tude of fine mossy processes, These features are shown in 
Fig. 30. 
