THE NERVOUS SYSTEM 267 
5. The trigeminus or fifth cranial nerve consists of motor 
and sensory portions. The latter arises from the trigeminal 
ganglion, the origin of which has already been described. The 
ganglionic rudiment appears roughly Y-shaped even at an early 
stage (cf. Figs. 105 and 117), the short stem lying against the 
wall of the brain and the two branches diverging one in the direc- 
tion of the upper surface of the optic cup (ophthalmic branch) 
and the other towards the mandibular arch. The original con- 
nection of the ganglion with the roof of the neural tube is lost 
during the second day and permanent connection is established 
during the third day, presumably by growth of axones into the 
wall of the brain. The new connection or sensory root of the 
trigeminus is attached to the myelencephalon in the region of 
greatest width of the fourth ventricle near the ventral portion 
of the lateral zone. 
During the fourth day the peripheral axones follow the direc- 
tion of the ophthalmic and mandibular branches of the ganglion 
and grow out farther as the ophthalmic and mandibular nerves; 
the former passes forward between the optic vesicle and the wall 
of the brain; the latter runs ventrally towards the angle of the 
mouth, over which it divides, a smaller maxillary branch entering 
the maxillary process of the mandibular arch, and a larger one, 
the mandibular nerve, runs into the mandibular arch. (For an 
account of the branchial sense organ of the trigeminus, see Chap. 
VI.) 
A medullary component of the trigeminal nerve arises from 
the wall of the brain just median to the ganglionic root during 
the fourth day; it runs forward parallel to the ganglionic ophthal- 
mic branch, and sends a twig to the ciliary ganglion. Beyond 
this point it unites with the ganglionic branch. 
A connection of the trigeminus with the olfactory sensory 
epithelium is described under the olfactory nerve. 
6. The sixth cranial or abducens nerve is stated to arise about 
the end of the fourth day. It is a purely motor nerve, and has 
no ganglion connected with it; it innervates the external rectus 
muscle of the eye. At 122 hours it arises by a number of slender 
roots attached to the myelencephalon near the mid-ventral line, 
beneath the seventh nerve. Its roots unite into a slender trunk 
that runs directly forward beneath the base of the brain to the 
region of the orbit. The sixth nerve thus corresponds more 
