84 PROFESSOR A, MILNES MARSHALL. 
their characteristic course and relations. The communicating 
branch from the fifth to the ciliary ganglion is undoubtedly the 
branch I have marked v d. The first of the two branches into 
which the nerve divides beyond the ganglion is the one marked 
in my figure 1114, while the second of these branches, the one 
which pierces the sclerotic, passes through the orbit, lying above 
the optic nerve, and finally runs to the front of the head, is, 
beyond all doubt, the nerve I have marked 11a. 
The nerve 12a, which runs through the orbit beneath the 
rectus superior and obliquus superior, and above the optic nerve, 
is the nerve which, in many Vertebrates, receives the name ramus 
ophthalmicus profundus of the fifth nerve. In most cases it 
has the appearance of a branch of the fifth nerve; this appear- 
ance I believe to be due to the communicating branch from the 
fifth (v d.) becoming directly continuous with the anterior branch 
of the third (111) and to the nerve thus formed losing its con- 
nection with the ciliary ganglion. Should this conjecture prove 
to be correct it will probably be found that in the early stages of 
development the nerve mia is connected with the ciliary 
ganglion, and that the connection is only lost comparatively 
late. 
Summary.—The third nerve, at stage K, arises from the mid 
brain by a single ganglionic root; it runs back to the interval 
between the first and second head cavities, where it expands into 
a ganglion—the ciliary ganglion. This ganglion receives a short 
communicating branch from the fifth nerve ; beyond it the nerve 
divides into two branches, of which one continues the course of 
the main stem, and ends in the oddiquus inferior, while the other 
runs forward through the orbit, and is the nerve usually described 
as the ramus ophthalmicus profundus of the fifth. My observa- 
tions leave no doubt in my mind that in Scydlium this nerve is 
really a branch of the third. Finally, in the later stages, a 
number of slender non-ganglionic roots of origin appear in front 
of the original ganglionic root. 
The Development of the Fifth (Trigeminal) Nerve—At the 
commencement of stage x the fifth nerve arises by a single root 
on either side from the lower part of the side of the hind brain. 
Immediately beyond its root of origin it expands to form a large 
ganglionic swelling—the future Gasserian ganglion—the lower 
part of which is wedged in between the dorsal ends of the second 
and third head cavities. From this ganglion three branches 
arise :—a. From the upper and anterior angle of the ganglion, 
close to its root of origin, a nerve arises which runs forwards 
and upwards and will be spoken of as the ophthalmic branch of 
the fifth. 4. From the lower and anterior part of the ganglion 
