266 THE DEVELOPMENT OF THE CHICK 



centralis, 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 (cf. 

 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: (a) 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 (5) 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 {h) 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). 



