the spinal canal by way of the foramen between the vertebra. The Cranial Nerves. — The cranial nerves consist of twelve pairs, as follows : I. Olfactory (fila). II. Optic. III. Oculomotor. IV. Trochlear. V. Trigeminal. VI. Abducent. VII. Facial. Nervus intermedins. VIII. Acoustic. 1. Cochlearis. 2. Vestibularis. IX. Glossopharyngeal. X. Vagus. XI. Spinal accessory. 1. Accessory to vagus. 2. Spinal part. XII. Hypoglossal. These nerves have each a superficial and a deep origin. The former corresponds to its point of attach- ment at the surface of the brain; the latter to certain nuclei or collections of nerve cells in the structure of the brain. The superficial origin only will be men- tioned. The Olfactory Nerves (First). — ^The olfactory nerves or fila are the special nerves of the sense of smell. Twenty in number on each side. They are distributed to the olfactory region in the upper part ANATOMY AND PHYSIOLOGY OF THE BRAIN 369 of the superior turbinated process of the ethmoid and corresponding portion of the nasal septum. These filaments represent the axones of the olfactory cells Fig. 131 r» mtal lobe. Occipital lobe. Base of brain, showing superficial origin of cranial nerves. and pass through the cribiform plate of the ethmoid bone to join the under surface of the olfactory bulb, which rests on the cribriform plate, and is the oval mass of a grayish color that forms the anterior extremity 24 '^ 370 THE NERVE SYSTEM of a slender process of brain substance, called the olfactory tract. The olfactory nerves differ in struc- ture from the other nerves, containing only amyelinic fibers. The Optic .Nerves (Second). — ^The fibers of the optic nerves, the special nerve of the sense of sight, are situated in the retina; they start as the central processes of the ganglion cells which converge and pierce the choroid and sclera as a cylindric cord. The point of emergence is situated a little internal to the posterior pole of the globe. Passing through the orbital fat, in an inward and backward direction, it passes through the optic foramen to end in the optic chiasm or commissure. The optic chiasm is somewhat quadri- lateral in shape, rests on the olivary eminence and the diaphragma sellse, being bounded above by the lamina terminalis; behind, by the tuber cinereum, on either side by the anterior perforated substance. Within the chiasm the fibers cross as follows: Those from the nasal side of the left and right halves of the retina cross in the centre, to the opposite optic tract; those from the temporal side of the right and left eyes pass backward without crossing, to end in the optic tract of the same side. The optic tract passes back to areas of the brain where the nerve impulses are conveyed to the cuneus (the area for the sense of sight in the cortex) by means of another pathway, the optic radiation. By connections with other nerve centres in the brain the optic tract communicates with the origin of the nerves which influence the muscles that control the movements of the eye-ball. The Oculomotor (Third). — The oculomotor arises superficially from the crus anterior to the pons, its deep origin being a gray nucleus in the floor of the aqueduct of Sylvius. It runs to the outer side of the posterior clinoid j)r()(PSs of the sphenoid bone, enters ANATOMY AND PHYSIOLOGY OF THE BRAIN 371 the cavernous sinus, runs above the other nerves in its outer wall, and divides into two branches, which enter the orbit between the two heads of the external rectus. It is joined in the sinus by sympathetic fila- ments. The superior branch crosses the optic nerve to supply the superior rectus and levator palpebrse muscles. The inferior divides into three parts — one for the inferior oblique, one to the inner, and one to the lower recti muscles. The first supplies the motor root of the lenticular ganglion of the sympathetic system. The Trochlear (Fourth). — The trochlear nerve has an apparent origin from the upper side of the valve of Vieussens, and a deep from the floor of the aqueduct of Sylvius. The two nerves communicate by a transverse band on the valve of Vieussens. The nerve pierces the dura after crossing over the crus, enters ANATOMY AND PHYSIOLOGY OF THE BRAIN 371 the cavernous sinus, in whose outer wall it lies between the ophthalmic and third nerves, then crosses the latter to enter the orbit through the sphenoidal fissure above the external rectus, and enters the superior oblique muscle. The Fifth Nerve (Trifacial). — The fifth or trifacial is the largest of all the cranial nerves, and arises by two roots, a motor and a sensor. The former is small, and the latter has the Gasserian ganglion upon it. Both arise from the side of the pons super- ficially, the smaller root above the larger, some trans- verse fibers of the pons separating the two. This nerve conveys both motion and sensation. At the apex of the petrous portion of the temporal the large root forms the Gasserian ganglion; the smaller does not join in the ganglion, but runs below it to join, just below the foramen ovale, the lowest trunk pro- ceeding from the ganglion. The Gasserian ganglion lies in a hollow near the apex of the petrous portion of the temporal bone, the large 372 THE NERVE SYSTEM superficial petrosal nerve, and the motor root lying below it. It receives branches from the carotid Fig. 132 Sensory root: Motor root. Auriculo-temporal tierpe. Distribution of the second and third divisions of the fifth nerve and submaxillary gangHon. (Gray.) plexus. Small twigs pass to the dura mater. This ganglion sends off three large branches, viz., the ophthalmic, superior maxillary, and inferior maxillary. ANATOMY AND PHYSIOLOGY OF THE BRAIN 373 The first two confer sensation, the third, motion and sensation. The ophthalmic nerve, or first division of the fifth nerve, is sensor and the smallest branch of the ganglion. It is flattened, about 1 inch long. It receives filaments from the cavernous plexus, and gives off filaments to the third and sixth, and sometimes to the fourth nerve, Fia. 133 Internal carotid artery and carotid plexus. Nerves of the orbit and ophthalmic ganglion. Side view. (Gray ) and a recurrent branch running in the tentorium cere- belli with the fourth. Finally, it divides into the frontal, lacrymal, and nasal nerves, which pass through the sphenoidal fissure into the orbit. The second division of the fifth nerve (superior maxillary) is sensor, and enters the foramen rotundum. 374 THE NERVE SYSTEM crosses the sphenomaxillary fossa, and, as the infra- orbital, traverses the canal, emerges from the foramen to end on the face in .the palpebral, nasal, and labial branches — the first set, to lower lid; the second, to side of nose; and the third, to upper lip. The orbital or temporomalar branch enters the orbit by the sphenomaxillary fissure, and divides into two branches, which pierce the malar bone. The alveolar or superior dental nerves are three: The posterior, middle, and anterior, which supply the upper row of teeth. The inferior maxillary nerve (third division of the fifth) is the largest branch, and arises by two roots — a large sensor root from the Gasserian ganglion and the motor root of the fifth. This nerve divides into two trunks, anterior and posterior. The anterior gives off the masseteric, the buccal, the deep temporal, and the two pterygoid nerves. The posterior trunk of the inferior maxillary is mostly sensory. It divides into the auriculotemporal, gustatory, and inferior dental; the last supplies the lower row of teeth. The gustatory or lingual nerve lies at first beneath the external pterygoid, internal to the dental nerve. Here a branch from the dental may cross the internal maxillary to join it. The chorda tympani also joins it. The nerve now runs along the inner side of the ramus of the jaw, and crosses the upper constrictor to the side of the tongue above the deep part of the submaxillary gland; lastly, it runs below Wharton's duct, and superficially along the side of the tongue to its apex. It communicates with the facial through the chorda tympani, the submaxillary ganglion, inferior dental, and hypoglossal. It supplies the mucous membrane of the mouth and tongue (anterior two-thirds), the gums, sublingual gland, and the ANATOMY AND PHYSIOLOGY OFTHE BRAIN 375 filiform and fungiform papillae in the mucous mem- brane on the back of the tongue. The Sixth Nerve. — The sixth or abducens has an apparent origin in the groove between the pons and medulla. It runs to the lower and outer part of the dorsum sellse, and traverses the floor of the cavernous sinus external to the carotid artery, and, receiving branches from the cavernous and carotid plexuses, enters the orbit by the sphenoidal fissure Frontal ^uperiot division oj oculo-mofor^ litferior division of ocido-motor. Ihdncent. Ophthalmic rein. Relations of structures passing through the sphenoidal fissure. (Gray.) between the two heads of the external rectus; it receives a branch from the ophthalmic nerve, and supplies the above-named muscles. The Seventh Nerve. — The seventh or facial has a superficial origin from the depression between the olivary and restiform bodies of the medulla oblon- gata. Between it and the eighth is the pars inter- media, which joins the facial in the auditory canal. The nerve runs outward to the internal meatus, where it runs in a groove on the auditory nerve, enters the aqueductus Fallopii, and emerges at the 376 THE NERVE SYSTEM Fia. 136 Terminaiiona stipratrochle infratrochle( of nasal. ^^^m\ ^ The nerves of the scalp, face, and side of the neck. (Gray.) ANATOMY AND PHYSIOLOGY OF THE BRAIN 377 stylomastoid foramen. It presents within the aque- duct, near the hiatus Fallopii, a reddish enlargement, the geniculate ganglion. Outside the cranium it runs forward in the parotid gland, and divides behind the ramus into the cervicofacial and temporofacial divisions. In the parotid and vicinity the radiating branches form the pes anserinus. The facial nerve supplies all the muscles of expres- sion of the face. The Auditory Nerve. — The eighth or auditory is the special nerve of the sense of hearing. Super- ficially it appears at the lower border of the pons, external to the facial. It has two roots — one from the inner side of and one from the front of the restiform body. It runs to the internal auditory meatus with the facial nerve, the two being separated by the pars intermedia and the auditory artery. The nerve in the meatus divides into a cochlear and a vestibular branch, whose distributions within the ear are described under the special sense of hearing. The Ninth Nerve. — The ninth or glossopharyngeal arises superficially by several filaments from the groove between the olivary and restiform bodies at the upper part of the medulla; deeply through the lateral tract to a gray nucleus in the floor of the fourth ventricle. The nerve runs through the middle part of the jugular foramen with the vagus and spinal accessory, in a separate sheath, and here presents two successive ganglionic enlargements, the jugular and the petrous ganglia. Outside the cranium it passes between the jugular vein and the internal carotid artery, descending in front of the latter, and beneath the styloid process and its muscles, to the lower border of the stylo- pharyngeus, and supplies the mucous membrane of the tongue. It then crosses this muscle and divides into branches beneath the hyoglossus. In the jugular 378 THE NERVE SYSTEM foramen it grooves the lower border of the petrous portion of the temporal bone. The Pneumogastric Nerve. — The tenth, vagus, or pneumogastric is both motor and sensor. Its apparent origin is by twelve to fifteen filaments below, and in the line of the origin of the ninth; its deep origin is from a nucleus in the lower part of the fourth ventricle. It passes through the jugular foramen in the same sheath with the spinal accessory, a par- tition separating them from the ninth, and develops the ganglion of the root of the vagus. Emerging from the foramen, it forms the ganglion of the trunk of the vagus. The ganglio7i of the root (ganglion jugulare) is gray in color and spherical, its diameter about two lines. It has branches of communication with the accessory part of the spinal accessory, w4th the petrous ganglion of the ninth, with the facial, and with the superior cervical ganglion of the sympathetic. The ganglion of the trunk (ganglion cervicale) is larger, of a reddish color, and cylindrical form. Its surface is crossed by the accessory portion of the eleventh, and it communicates with the hypoglossal, the upper two cervical, and the sympathetic nerves. The vagus then descends between the internal carotid artery and the jugular vein to the thyroid cartilage, then between the vein and the common carotid to the root of the neck; where it enters the thorax and gives off branches to the heart (cardiac plexuses), lungs (pulmonary plexuses), and esophagus; it then passes through the esophageal opening in the diaphragm to enter the abdominal cavity w^here it gives off branches to the solar plexus, the stomach, liver, spleen, kidneys, suprarenal glands, and pan- creas. The Eleventh Pair. — The eleventh, or spinal accessory, consists of a spinal portion and an accessory ANATOMY AND PHYSIOLOGY OF THE BRAIN 379 part to the vagus. The latter part arises as five or six filaments from the lateral tract of the medulla, below the origin of the vagus. The spinal portion arises from the lateral column of the cord as low as the sixth cervical nerve, the fibers being connected with the anterior horn of gray matter. This part then ascends, between the posterior nerve roots and the ligamentum denticulatum, through the foramen magnum, then out again by the jugular foramen, lying in the sheath of the vagus, and here communicates with the accessory portion. After its exit from the skull it crosses the internal jugular vein and pierces the sternomastoid to end in the trapezius muscle. The Hypoglossal. — The twelfth, or hypoglossal, nerve arises by ten to fifteen filaments from the groove between the pyramid and olivary body in the medulla. The deep origin is from a nucleus in the floor of the fourth ventricle. The filaments form two bundles which pierce the dura separately and unite in the anterior condylar foramen. The nerve descends behind the internal carotid artery and internal jugular vein, closely bound to the vagus. ^ The Spinal Nerves.— The spinal nerves consist, on each side, of eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal, in all thirty-one pairs, which arise from the cord by two roots, anterior and posterior. The latter are the larger, and are supplied with ganglia. The suboccipital or first cervical nerve has no ganglion. The two roots unite just beyond the ganglion, and the resulting trunk divides into two divisions, anterior and posterior, each containing fibers from both roots, sensor and motor. The posterior division divides into an external and an internal branch. The anterior divisions in the dorsal region remain separate, but elsew^here they unite into plexuses. ' See Fig. 131 for origin of spinal nerves. 380 THE NERVE SYSTEM They are larger than the posterior. Each division is connected with the sympathetic gangha along the vertebral column, by means of nerve trunks called rami communicantes. (See Sympathetic System, p. 385.) The Cervical Plexus. — The cervical plexus is formed by the anterior divisions of the upper four cervical nerves, which emerge between the scalenus medius and rectus anticus major. It lies upon the scalenus medius and levator anguli scapulae, beneath the sternomastoid. Each nerve except the first divides into a branch for the nerve above and one for the nerve below. The anterior division of the first (suboccipital) nerve grooves the atlas beneath the vertebral artery, and joins the second, supplying the rectus lateralis and recti antici muscles. It commu- nicates with the sympathetic vagus, and hypoglossal nerves. Its branches are superficial and deep. The superficial are divided into ascending and descending. Ascending Branches. — (a) The superficialis colli, (b) auricularis magnus (great auricular), (c) occipital minor. Descending Phrenic, from the third, fourth, and fifth, descends on the scalenus anticus, then between the subclavian artery and vein, and crosses the internal mammary artery. It then crosses in front of the root of the lung and runs between the pericardium and mediastinal pleura to the diaphragm. The Brachial Plexus. — The brachial plexus is formed by the anterior divisions of the lower four cervical and first thoracic, as follows: The fifth and sixth form an upper; the seventh, a middle; and the eighth cervical with first dorsal a lower trunk. Each of these trunks then separates into an anterior and a posterior branch. ANATOMY AND PHYSIOLOGY OF THE BRAIN 381 The anterior branches of the upper and middle trunks form the outer cord of the plexus; the anterior branch of the lower, the inner cord; of the posterior cord it is variously stated that the posterior branches of all three trunks form it, or that the posterior branches of the upper and middle trunks form it, while the Fig. 136 C. with Phrenic VII C^vical Anterior division o} Middle Ti-unk Anterior division of Upper Trunk External Anterior Thoracic Posterior division of Upper 2'runk Upper Sub-scapular Middle and Lower Sub-scapular Lower Trunk Posterior Thoracic Anterior division of Lower Trunk Posterior division of Lower Trunk Internal Anterior Thoracic Lesser Internal Cutaneous' Internal Cutaneous' Musculo-cutaneous Ulnars Museulo-spiral'' Plan of the brachial plexus. (Gray.) posterior branch of the lower trunk joins the musculo- spiral nerve. It is altogether a matter of dissection. The plexus is at first between the anterior and middle scaleni, then above and external to the subclavian artery. It passes behind the clavicle and subclavius, lying on the subscapularis and serratus magnus muscles. The cords lie external to the first part of the axillary 382 THE NERVE SYSTEM artery, but
Key Takeaways
- Understanding cranial and spinal nerve origins is crucial for diagnosing neurological issues.
- The facial nerve (VII) supplies muscles of expression, which can be vital in assessing patient conditions.
- Knowledge of the vagus nerve (X) helps in managing respiratory and digestive functions during emergencies.
Practical Tips
- Identify key cranial nerves that control critical functions like vision, smell, and facial expressions to quickly assess a patient's neurological status.
- Be aware of the phrenic nerve (C3-C5), which innervates the diaphragm; its injury can lead to respiratory failure.
- Recognize the importance of the vagus nerve in managing heart rate and digestion during stress or trauma.
Warnings & Risks
- Injuries to cranial nerves, especially those controlling vital functions like breathing and swallowing, can be life-threatening.
- Damage to spinal nerves can result in paralysis; immediate medical attention is necessary.
- Misdiagnosis of nerve injuries can lead to improper treatment and worsen the patient's condition.
Modern Application
While this chapter provides foundational knowledge on cranial and spinal nerves, modern emergency responders should also be familiar with advanced diagnostic tools like MRI and CT scans. However, understanding these nerves is still crucial for quick assessments in remote or resource-limited settings.
Frequently Asked Questions
Q: What are the functions of the olfactory nerve (I)?
The olfactory nerve (I) is responsible for the sense of smell. It contains only amyelinic fibers and has 20 filaments on each side, which are distributed to the olfactory region in the upper part of the superior turbinated process of the ethmoid bone.
Q: How does the optic nerve (II) transmit visual information?
The optic nerve (II) starts as central processes of