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 



