ory labyrinth without injury is a difficult performance, but
its structure may be made out very satisfactorily by paring away
successive slices of the otic mass. Such a section is shown by
Figure 3b; through the translucent hyaline cartilage the utriculus and
horizontal canal can be darkly seen. The ductus endolymphaticus
(vide Rabbit) is indicated by a dotted line in our figure. It is situated
internal to the right-angle between the two vertical canals, and
reaches to the surface of the otic capsule.

Section 14. The brain shows the three primary vesicles much more
distinctly than do our higher types. The fore-brain has large laterally
separated olfactory lobes (rh.), there are relatively small
"hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and
the gland itself is much nearer the surface, being embedded in the
cartilage of the brain case, and the pituitary body is relatively very
large, and has lateral vascular lobes on either side. Following the
usual interpretation of the parts, we find optic lobes (op.l.) as the roof
of the mid-brain, and behind a very large, median, hollow,
tongue-shaped cerebellum (c.b.). The medulla is large, and certain
lateral restiform tracts (r.t.) therein, which also occur in the higher
types, are here exceptionally conspicuous.

Section 15. The dog-fish has ten pairs of cranial nerves,
corresponding to the anterior ten of the rabbit very closely, when we
allow for the modification the latter has suffered through the
conversion of some part of the spiracular cleft to an eardrum, and the
obliteration of the post-hyoid branchial slits.

The first and second nerves are really brain lobes, and nerves of the
special senses of smell and sight respectively.

The third (oculomotor), the fourth (patheticus), and the sixth
(abducens) are distributed to exactly the same muscles of the eyeball
as they are in the rabbit.

The fifth nerve, has, in the dog-fish, as in the rabbit, three chief
branches. V.2 and V.3 fork over the mouth just as they do in the
mammal; V.1 passes out of the cranium by a separate and more
dorsal opening, and runs along a groove along the dorsal internal wall
of the orbit, immediately beneath a similar branch of VII., which is not
distinct in the rabbit. The grooves are shown in the figure of the
cranium, Sheet 18; the joint nerve thus compounded of V. and VII. is
called the ophthalmic (oph.). It is distributed to the skin above the
nose and orbit. W