266 THE DEVELOPMENT OF THE CHICK 
ventralis, and ending in the mesenchyme a short distance from 
its point of origin. At seventy-two hours the root is much 
stronger, interpenetrated with mesenchyme and ends between 
the optic cup and floor of the brain behind the optic stalk (ef. 
Fig. 101). At ninety-six hours the root is broad and fan-shaped, 
the nerve itself is comparatively slender, and passes downwards 
and backwards behind the optic-stalk where it enters a well- 
defined ganglion situated just median to the ophthalmic branch 
of the trigeminus; this is the ciliary ganglion; beyond it the 
fibers of the oculo-motor turn forward again to enter the region 
of the future orbit. 
According to Carpenter (1906) the ciliary ganglion arises 
from two sources: (@) migrant medullary neuroblasts that pass 
out into the root of the oculo-motor, and follow its course to 
the definitive situation of the ciliary ganglion, and (b) a much 
smaller group of neuroblasts that migrate from the ganglion of 
the trigeminus along the ophthalmic branch, and by way of a 
ramus communicans to the ciliary ganglion. The adult ciliary 
ganglion shows correspondingly two component parts: (a) a 
larger ventral region composed of large bipolar ganglion cells, 
and (6b) a smaller dorsal region containing small ganglion cells 
with many sympathetic characters. It is probable that the 
medullary fibers of the oculo-motor nerve are distributed entirely 
to the muscles innervated by it, viz., the superior, inferior, and 
internal rectus and inferior oblique muscles of the eye. The 
fibers arising from the neuroblasts of the ciliary ganglion ter- 
minate peripherally in the intrinsic muscles of the eye-ball, and 
centrally (in the case of the bipolar cells) in the brain, which 
they reach by way of the medullary nerve. The motor branches 
leave the trunk of the nerve a short distance centrally to the 
ciliary ganglion. 
4. The trochlearis or fourth cranial nerve is peculiar inas- 
much as it arises from the dorsal surface of the brain in the 
region of the isthmus. It arises entirely from medullary neuro- 
blasts and innervates the superior oblique muscle of the eye. 
Marshall states that it may be readily seen in a five-day embryo; 
in an embryo of eight days it is a slender nerve arising from the 
dorsal surface of the isthmus immediately in front of the cere- 
bellum; the fibers of the two sides form a commissure in the roof 
of the isthmus (Fig. 148). 
