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external rectus muscle of the eye. causes inward squint.
Paralysis or section of it
VII, (PORTIO DURA) MOTOR NERVE OF THE FACE. This nerve arises from a gray nucleus in the floor of the fourth ventricle. It passes, with the other part of the seventh (portio mollis) or auditory nerve, into the internal auditory meatus of the temporal bone. It first passes out toward the hiatus, then turns at a right angle to form a knee-like swelling (geniculate ganglion), and then runs backward along the top of the inner wall of the drum, and passing downward through a special canal in the bone, comes out at the stylo-mastoid foramen, and finally spreads out on the side of the face. It is essentially an efferent nerve, being partly motor and partly secretory, though its connections have caused afferent functions to be ascribed to it. Its distribution may be thus briefly summarized :
(i) Motor Fibres.-(1) To the muscles of the forehead, eyelids, nose, cheek, mouth, chin, outer ear and the platysma, which may be grouped together as the muscles of expression, (2) To some muscles of mastication, viz., buccinator, posterior belly of digastric, and the stylohyoid-all the foregoing being supplied by external branches—while in the temporal bone it gives a branch to (3) the stapedius muscle, and also a branch from the geniculate ganglion, named the great superficial petrosal nerve, which, after a circuitous course, is supplied to the elevator and azygos muscles of the palate and uvula.
(ii) Secretory Fibres.-(1) To the parotid gland by the small superficial petrosal nerve,
which sends a branch to the otic ganglion, whence the fibres pass to the auriculo-temporal nerve, and then on to the gland. (2) To the submaxillary gland by the chorda tympani, which, after having traversed the tympanum, leaves the ear by a fissure at its anterior extremity, then joins the lingual branch of the fifth to separate from it and pass into the submaxillary ganglion, which lies in close relation to the gland (compare Figs. 64 and 65).
(iii) Vasomotor or vaso-inhibitory influences are chiefly connected with the motor and secretory functions, since dilatation of the vessels of muscles and glands accompanies the motion and secretion that follows stimulation of the nerves going to them.
(iv) The following afferent impulses are said to travel along the track of the portio dura and its branches: (1) Special taste sensations, which are chiefly located in the chorda tympani branch, may be explained by the branches of communication which pass from the trunk and petrous ganglion of the glossopharyngeal to the portio dura at its exit from the foramen, or by the connection in the drum of the ear between the tympanic branch of the glosso-pharyngeal and the geniculate ganglion of the portio dura through the lesser superficial petrosal nerve. (2) Ordinary sensations, which are also located in the chorda tympani, are said to traverse this nerve in an afferent direction until it comes near the otic ganglion, when the sensory fibres leave the chorda and pass to the inferior division of the fifth nerve through the otic ganglion.
Injury of the facial nerve in any of the deeper parts of its course gives rise to the striking group of symptoms known as facial paralysis, the details of which are too long to be given here. When it is remembered that muscles aiding in expression, mastication, deglutition, hearing, smelling, and speaking are paralyzed, and that taste, salivary secretion, and possibly ordinary sensation are impaired, one can form some idea of the complex pathological picture such a case presents.
V, N. TRIGEMINUS, OR TRIFACIAL NERVE. This nerve transmits both efferent and afferent impulses carried by two different strands of fibres. The motor part, which arises from a gray nucleus in the floor of the fourth ventricle, is much the smaller of the two, and has been compared to the anterior root of a spinal nerve. The large sensory division springs from a very extensive tract, which can be traced from the pons Varolii through the medulla to the lower limit of the olivary body, and on to the posterior cornua of the spinal marrow. This set of fibres has been linked to the posterior root of a spinal nerve, being somewhat analogous to it in origin, function, and the fact that there is a large ganglion on it within the cranium.
The distribution and peripheral connections of this nerve are somewhat complicated, and should be carefully studied when the manifold functions of its branches are being considered. The various impulses conveyed by the trifacial nerves may be thus enumerated :
(1) EFFERENT FIBRES. 1. Motor.–To the muscles of (1) mastication, viz., temporal masseters, both pterygoids, mylohyoid, and the anterior part of the digastrics; (2) to the tensor muscle of the soft palate ; and (3) to the tensor tympani. (4) In some animals (rabbit) nerve filaments are said to pass to the iris, reaching the eyeball by the ciliary ganglion.
2. Secretory.—The efferent impulses which stimulate the cells of the lachrymal gland to increased action pass along the branches of the ophthalmic division of this nerve.
3. Vasomotor.—The nerves governing the muscles of the blood vessels of the eye, of the lower jaw, and of the mucous membrane of the cheeks and gums.
4. Trophic.-On account of the impairment of nutrition of the eye and mucous membrane of the mouth, which occurs after injury of fifth nerve, it is said to carry fibres which preside over the trophic arrangements of these parts.
(2) AFFERENT FIBRES. 1. Sensory. -All the divisions of the trifacial nerve may be said to be connected with cutaneous nerves, by which the ordinary sensory impulses are carried from—(1) the entire skin of the face, and the anterior surface of the external ear ; (2) from the external auditory meatus ; (3) from the teeth and periosteum of the jaws, etc. ; (4) from the mucous membrane lining the cheeks, floor of the mouth, and anterior part of the tongue ; (5) from the lining membrane of the nasal cavity; (6) from the conjunctiva, ball of the eye, and orbit generally ; (7) and from the dura mater, including the tentorium.
2. Excito-motor.—Some of the fibres which have just been enumerated as carrying ordinary sensory impressions have special powers of exciting coördinated reflex motions. Thus the sensory fibres from the conjunctiva and its neighborhood are the afferent channels in the common reflex acts of winking and closing the eyelids; and the fibres from the nasal mucous membrane excite the involuntary act of sneezing.
3. Excito-secretory.—As in the case of reflex movements, secretion may be excited reflexly. Fibres carry afferent impulses to the medulla from the anterior part of the tongue, and excite activity of the salivary glands. Stimulation of the mucous membrane of the nose or eye causes impulses to pass to the secretory centre of the lachrymal glands, which are frequently thus reflexly excited.
Intense stimulation of almost any of the afferent nerves may excite these reflex phenomena. Thus the most stoic person will experience active secretions of saliva and lachrymal fluid, as well as spasmodic closure of the lids during the extraction of a tooth. Even the bold use of a blunt razor will cause tears to flow down the cheeks, by sending excito-secretory impulses along the branches of the inferior and superior maxillary division of this
4. Tactile impulses are appreciated by the anterior part of the tongue with remarkable delicacy, and are conveyed by the lingual branch of the fifth nerve ; most of the cutaneous fibres are also capable of receiving tactile stimulation.
5. Taste.—The tastes appreciated by the anterior part and the edges of the tongue are carried by fibres which lie in the peripheral branches of this nerve. These belong chiefly, if not altogether, to the chorda tympani, and leave this lingual branch of the fifth to join the seventh nerve on their way to the trunk of the glosso-pharyngeal.
There are four ganglia in close relation to the branches of the fifth nerve which have certain points of similarity, and may, therefore, be considered together, although their positions show that they are engaged in the performance of very different functions.
We have not yet been able to ascertain the value of these little points of junction of motor, sensory, vasomotor, and secretory fibres, because, so far, we are unable to attribute to the cells of the ganglia either reflecting or controlling action, or any automatic power.
They all have efferent (motor and secretory) and afferent (sensory) connections with the nervous centres, and also connections with the main channels of the sympathetic nerves. These are spoken of as the roots of the ganglia. Their little branches are generally mixed nerves.
THE CILIARY OR OPHTHALMIC GANGLION. This ganglion lies in the orbit. It has three roots, which come from—(1) the inferior oblique branch of the third nerve, by a short slip, which forms the motor root ; (2) from the nasal branch of the ophthalmic division of the fifth, and (3) from the carotid plexus of the sympathetic. The branches go mostly to the ball of the eye, and may be divided into afferent and efferent. The afferent are sensory branches, connecting the cornea and its neighboring conjunctiva with the centres. The efferent fibres go to the iris and cause dilatation of the pupil (coming mostly from the sympathetic), and the vasomotor fibres going to the choroid coat, iris, and retina.
THE SPIENO-PALATINE OR NASAL GANGLION. This lies on the second division of the fifth nerve, from which it gets its sensory root. Its motor root comes from the seventh by the great superficial petrosal nerve, and its sympathetic root from the carotid plexus by the branch joining this nerve. These enter the ganglion together, and are spoken of as the vidian
Afferent (sensory) impulses, from the greater part of the nasal cavity, pass through this ganglion. Its efferent branches are-(1) motor to the elevator of the soft palate and azygos uvulæ ; (2) vasomotor, which comes from the sympathetic; and (3) secretory, which supply the glands of the cheek, etc.
OTIC OR EAR GANGLION. The otic ganglion lies under the foramen ovale, where the interior division of the fifth comes from the cranium. Its roots are—(1) motor ;(2) sensory, from the inferior division of the fifth ;