The trigeminal nerve is the fifth cranial nerve. It has three divisions: the ophthalmic, maxillary, and man
dibular divisions. It is the principal sensory nerve of the head, supplying: (1) the skin of the face and an
terior half of the head; (2) the mucous membrane of the nose, paranasal air sinuses, mouth, and anterior twothirds of the tongue; (3) the teeth and temporo
mandibular joint; (4) the contents of the orbit, ex
cept the retina; and (5) parts of the dura mater.
DISSECTION 8.10 Middle cranial fossa Objectives
I. To remove the dura from the cavernous sinus and the rest of the middle cranial fossa. II. To identify and trace the structures in the cavernous sinus. III. To open the cavum trigeminale and dissect the trigeminal ganglion and its branches. IV. To trace the greater and lesser petrosal nerves.
Instructions
1. Turn each half of the tentorium anterolaterally on its attachment to the petrous temporal bone.
2. Find the trochlear nerve [Fig. 8.10]. It pierces the inferior surface of the tentorium, close to its free border and near the apex of the petrous temporal bone.
3. Inferior to the trochlear nerve, find the large trigem- inal nerve [Figs. 8.10, 8.14]. Pass a blunt probe for- wards along the nerve. The probe enters the dural sac, the cavum trigeminale which surrounds the nerve, and the trigeminal ganglion underneath the
dural floor of the middle cranial fossa. Elevate the probe to raise the dural floor of the middle cranial fossa, and outline the position of the nerve and gan- glion [Fig. 8.15].
4. Carefully remove the dura mater from the floor of the middle cranial fossa by cutting through it on the probe in the cavum trigeminale.
5. Strip the dura forwards and laterally to uncover the trigeminal nerve, the ganglion, and the three large branches of the nerve [Fig. 8.15]. The branch- es arise from the convex anterior border of the ganglion. Remove the dura from the ganglion with care, because the ganglion is a loose mass of cells which is easily destroyed.
6. Trace the mandibular nerve inferolaterally to the foramen ovale, close to the entry of the middle meningeal artery.
7. Follow the maxillary nerve to the foramen rotundum.
87
Middle cranial fossa
8. Follow the ophthalmic nerve into the lateral wall of the cavernous sinus where it divides into three branches which can be traced to the superior or- bital fissure [Fig. 8.14].
9. Find the trochlear and oculomotor nerves in the lateral wall of the cavernous sinus. Pick them up as they pierce the dura, and gently pull on them to identify their more peripheral parts.
10. Preserve the dura where the nerves enter it, but remove it in front of that point so as to follow the nerves to the superior orbital fissure.
11. Remove the remains of the lateral wall of the cav- ernous sinus from around the nerves, and expose the internal carotid artery and the abducens nerve within the sinus. Note that the nerve passes
forwards, lateral to the artery. Follow the nerve forwards and backwards.
12. Carefully strip the remainder of the dura from the anterior surface of the petrous temporal bone, and look for the greater and lesser petrosal nerves.
These slender nerves emerge through slits in the temporal bone and run anteromedially in shallow grooves. The lesser petrosal nerve pierces the skull near the foramen ovale; the greater petrosal nerve is larger and more medial. It disappears under the trigeminal ganglion [Fig. 8.2B].
13. Lift the trigeminal ganglion, and try to identify the motor root of the trigeminal nerve on its inferior surface. It runs deep to the ganglion to the foramen ovale.
Fig. 8.13 (A) Transverse section through the head at the level of the pituitary gland. (B) Axial contrast computerized tomography (CT) through the pituitary (asterisk) and orbit.
Lens Ethmoid air sinus
Sphenoid air sinus
Internal carotid artery Pituitary
Tentorium cerebelli, free margin Eyeball
Lateral rectus Medial rectus
(A)
Eyeball
Ethmoidal air sinus Internal carotid artery Basilar
artery
Tentorium cerebelli, free margin
(B)
*
88
The cranial cavity
Levator palpebrae superioris
Trochlea
Superior oblique
Medial rectus Inferior rectus
Trochlear N.
Abducens N.
Optic N.
Oculomotor N.
Trochlear N.
Abducens N.
Mandibular N.
Superior rectus Superior
oblique Lacrimal gland
Lateral rectus Ciliary ganglion Nasociliary N.
Ophthalmic N.
Maxillary N.
Motor root, trigeminal N.
Trigeminal ganglion
Fig. 8.14 Orbit and middle cranial fossa. The trigeminal nerve and ganglion have been turned laterally to expose the motor root.
Fig. 8.15 Diagram to show the relations of the meninges to: (A) a spinal nerve; and (B) the trigeminal ganglion in the cavum trigeminale.
Arachnoid Dura mater Spinal ganglion
(A)
Trigeminal ganglion in cavum trigeminale
Dura mater
Mouth of cave Arachnoid
Periosteum of middle cranial fossa (B)
The motor fibres arise in the pons and pass into the mandibular nerve. They supply the four muscles of mastication—the temporalis, masseter, and medial and lateral pterygoids—and the mylo
hyoid, anterior belly of the digastric, tensor palati, and tensor tympani.
Almost all of the sensory fibres in the trigemi
nal nerve are processes of the cells in the large trigeminal ganglion. The central processes converge
to form the sensory root of the nerve which enters the pons. The peripheral processes enter the oph
thalmic, maxillary, and mandibular nerves. Proprio
ceptive nerve fibres from the same three nerves pass through the ganglion to cells in the midbrain—a unique arrangement where primary sensory nerve cells have their cell bodies within the central ner
vous system. (The sensory and motor nuclei of the trigeminal nerve are described in Chapter 28.)
89
Middle cranial fossa