THE TRIGEMINAL NERVE 795 
immediately above the foramen orbitale or through the latter and passes forward 
along the medial wall of the orbit to end in the posterior part of the dorsal oblique 
muscle of the eyeball (Fig. 564). 
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 backward in the tegmentum, 
then turn dorso-medially, and undergo total decussation with those of the opposite nerve in the 
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 TRIGEMINAL NERVE 
The trigeminal nerve (N. 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. 629). 
The sensory root (Portio major) extends forward through a notch on the lower 
part of the petrosal crest and widens out to join the semilunar ganglion. 
The semilunar ganglion (Ganglion semilunare)? is a crescent-shaped mass of 
nerve-fibers and cells which overlies the antero-lateral part of the foramen lacerum 
basis cranii, and is partly embedded in the dense fibrous tissue which occupies the 
foramen except where vessels and nerves pass through. Its long axis, which is 
about an inch (2.5 em.) in length, is directed forward and medially and its convex 
anterior face gives rise to the ophthalmic, the maxillary, and the sensory part of 
the mandibular division of the nerve. The surface of the ganglion is irregularly 
striated. It is connected by filaments with the adjacent carotid plexus of the sym- 
pathetic, and sends delicate twigs to the dura mater. 
The fibers of the sensory root arise from the ganglion as axones of the ganglion cells, and the 
fibers of the nerves which extend peripherally from the ganglion are dendrites of the cells. The 
sensory root-fibers enter the tegmentum of the pons and divide into anterior and posterior branches, 
which terminate about the cells of the sensory nucleus of termination of the trigeminus. This 
nucleus extends from the pons to the sixth cervical segment of the spinal cord (Dexler). The 
posterior branches of the fibers are collected into a compact bundle, the spinal tract or root of the 
trigeminus, which lies lateral to the substantia gelatinosa in the medulla. The central connections 
of the sensory part of the trigeminus are very extensive. The most important paths are: (1) 
Axones of cells of the sensory nucleus and the substantia gelatinosa pass chiefly as arcuate fibers 
across the raphé to the thalamus, whence impulses are transmitted by thalamo-cortical fibers 
to the cerebral cortex. In ungulates a distinct tract extends from the anterior part of the sensory 
nucleus to the thalamus of the same side (Wallenberg). It is probable that collaterals of the 
arcuate fibers go to the motor nuclei of the fifth, seventh, ninth, and tenth cranial nerves. (2) 
Axones of cells of the sensory nucleus enter the posterior cerebellar peduncle of the same side 
and reach the cerebellar cortex. (3) Collaterals are distributed to the nuclei of origin of the 
hypoglossal and of the motor part of the trigeminal and facial nerves. 
The motor root (Portio minor) extends forward beneath the sensory root and 
the semilunar ganglion and is incorporated with the mandibular division of the 
nerve. Its fibers arise chiefly from the so-called masticatory nucleus, which is 
situated in the pons near the inner face of the sensory nucleus; a few of these fibers 
come from the nucleus of the opposite side and cross in the raphé. Other fibers, 
which constitute the mesencephalic root, arise from cells in the outer part of the 
central gray matter of the mid-brain. 
It is evident from the foregoing statements that the trigeminus has essentially 
the same arrangement as a typical spinal nerve. It divides into three branches. 
I. The ophthalmic nerve (N. ophthalmicus) (Figs. 563, 564, 565) is purely 
sensory and is the smallest of the three branches of the trigeminus. It arises from 
the medial part of the front of the semilunar ganglion, passes forward along the outer 
side of the cavernous sinus, and is blended with the maxillary nerve for some dis- 
tance. It enters the foramen orbitale with the third and sixth nerves and divides 
into three branches. 
1. The lacrimal nerve (N. lacrimalis) runs forward on the rectus dorsalis and 
1 Also known as the trifacial nerve. 2 Formerly termed the Gasserian ganglion. 
