THE FIFTH, TRIGEMINAL, OR TRIFACIAL NERVE 677 



the optic papilla, where they are collected into a round trunk, the optic nerve. The 

 nerve thus formed ])ierces the chorioid and sclera, emerges from the pos- 

 terior part of the eyeball, and passes backward and inward to the optic foramen. 

 After traversing the latter it decussates with its fellow of the opposite side to form 

 the optic chiasma or commissure. In the orbit the nerve is slightly flexuous and is 

 embedded in the fat behind the eyeball and surrounded by the retractor oculi 

 muscle. Its intraosseous part is an inch or more (ca. 3 cm.) long. The sheath 

 of the nerve is formed by prolongations of the membranes of the brain, and includes 

 continuations of the subdural and subarachnoid spaces. 



The greater part of the fibers of the optic nerve cross in the chiasma to the tract of the oppo- 

 site side. In the tract the fibers proceed to (1) the internal geniculate body, (2) the posterior 

 part of the thalamus, and (3) the anterior quadrigeminal body (indirectly). The fibers which 

 go to the internal geniculate body appear to belong to CJudden's commissure and to be non- 

 visual in function. The visual fibers, which come from the outer part of the retina of the same 

 side and the inner part of the retina of the opposite side, terminate about cells in the anterior 

 quadrigeminal body and the part of the thalamus which corresponds to the pulvinar and external 

 geniculate body of man. From the cells of the former fibers pass to the nuclei of the motor 

 nerves of the eyeball, and complete the reflex arc. Fibers proceed from the cells of the thalamus 

 to the visual area of the cortex in the occipital part of the hemisphere. 



THE THIRD OR OCULOMOTOR NERVE 

 The oculomotor nerve (X. oculomotorius) arises by several radicles from the 

 basal surface of the cerebral peduncle, a little lateral to the interpeduncular furrow. 

 It turns sharply outward and forward, crosses over the cavernous sinus, and con- 

 tinues above the maxillary nerve and in company with the ophthalmic nerve to the 

 foramen lacerum orbitale. It emerges through the foramen with the latter nerve 

 and the abducens and divides into two l^ranches. The superior branch is short and 

 divides into twigs which supply the rectus superior, retractor, and levator pal]5el)ra! 

 superioris. The inferior branch (Figs. 438, 439) is larger and much longer. It 

 supplies the motor fibers to the ciliary ganglion (which lies directly on this branch 

 in the horse) and short branches to the rectus internus and rectus inferior, and 

 continues forward on the latter to end in the inferior oblique muscle. 



The deep origin of the fibers of the oculomotor nerve is in the oculomotor nucleus situated 

 in the gray matter of the floor of the cerebral aqueduct in the region of the anterior corpora 

 quadrigemina. 



THE FOURTH OR TROCHLEAR NERVE 

 The trochlear or pathetic (N. trochlearis) is the smallest of the cranial nerves. 

 It arises from the anterior cerebellar peduncle just behind the corpora quadrigemina, 

 curves outward and forward, pierces the tentorium cerebelli, and passes forward 

 along the outer border of the maxillary nerve (Figs. 504, 505). It emerges from 

 the cranium through a small foramen immediately above the foramen lacerum 

 orbitale or through the latter and passes forward along the inner wall of the orbit 

 to end in the posterior part of the superior oblique muscle of the eyeball (Fig. 439). 



The fibers of the fourth nerve spring from a nucleus in the gray matter of the floor of the 

 cerebral aqueduct behind the oculomotor nucleus. The fibers run luickward in the tegmentum, 

 then turn upward and inward and undergo total decussation with those of the opposite nerve in 

 tlic anterior part of the anterior medullary velum. In addition to this peculiarity it is the only 

 nerve which is connected with the dorsal aspect of the brain. 



THE FIFTH, TRIGEMINAL. OR TRIFACIAL NERVE 

 The trigeminal nerve (X. trigeminus) is the largest of the cranial series. It 

 is connected with the lateral part of the pons by a large sensory root and a smaller 

 motor root (Fig. 499). 



