86 PROFESSOR A, MILNES MARSHALL, 
The several branches of the fifth nerve are shown at stage M 
by the series of figs. 15 to 20, of which, as already noticed, 
fig. 15 is the most superficial, and fig. 20 the deepest. In 
figs. 16 and 17 the maxillary (v 4.) and mandibular (vc.) branches 
are shown very clearly ; these two nerves lie close to the surface, 
though, as we shall see immediately, the maxillary is separated 
from the surface by the palatine branch of the seventh. In 
fig. 19 the communicating branch (v d.) between the fifth and 
third nerves is shown lying at a deeper level than the maxillary and 
mandibular nerves; it is a short nerve connecting the Gasserian 
and ciliary ganglia directly together. The same figure shows 
also two portions of the ophthalmic branch (v a.) of the fifth, 
which runs forward close to the dorsal surface of the head and 
immediately beneath the superficial epiblast. In fig. 20, taken 
from the same embryo as fig. 19, but from the opposite side, the 
root of the ophthalmic nerve (Vv a.) and the communicating branch 
(v d.) are well seen; the latter is seen to give off a fine branch 
(v e.), which runs upward to the muscle marked o. s. 
Fig. 24 shows the roots of the ophthalmic and communicating 
branches in horizontal section; the former (v a.) is seen to be 
very close to the surface, while the latter (v d.) is situated 
more deeply. 
Figs. 26 to 30 represent a series of transverse sections 
through the head of an embryo between stages m and wn; they 
serve to illustrate the above description from a different point of 
view. The sections are all slightly oblique, so that in each sec- 
tion the left-hand half is in a plane a little posterior to the right- 
hand half. Fig. 26 passes through one of the anterior non- 
ganglionic roots of the fifth (v “), it also shows on the left 
side the root of the ophthalmic nerve (va.), and the whole 
length of the communicating branch (vd.) running to the 
ciliary ganglion (¢c.g.). On the right hand side the ophthalmic 
nerve (V a.) is seen in section. 
Fig. 27 passes on the left side through the main or ganglionic 
root of the fifth; on the right side it corresponds to the left side 
of fig. 26. Fig. 28 shows on the left side the maxillary nerve 
(v 4.) in nearly the whole of its length. 
Finally, in figs. 32 to 35 the condition and relations of the 
ophthalmic branch at stage o are shown; it is seen to be a 
slender nerve running forward along the dorsal surface to the 
fore part of the head, and giving off branches (fig. 32) on its 
course. 
Summary.—At stage kK the fifth nerve arises from the hind 
brain by a single large ganglionic root , it expands into a large 
ganglionic swelling, from which three nerves arise : (a), the oph- 
thalmic branch, which runs along the dorsal surface to the 
