(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
Some striated muscles are included within the skin of the facial mask. They are responsible for specific movements of the two palpebral openings of orbits and of the mouth. They are activated by the facial nerve (VII), but their coordination is made by the skin innervated by the trigeminal nerve (V). The technical problem of the superior orbital floor is linked to the trophicity of the cornea, which has to be perfectly transparent and, hence, cannot receive a vascular network. This explains the need to have a double trophic liquid space: one on the internal side given by the aqueous humour permanently secreted and resorbed in the anterior chamber and the other one on the external surface, in contact with air. This arrangement justifies the precise and efficient organisation of the lacrimal system, which must maintain a liquid film on the cornea by moving periodically the eyelids with the orbicular muscle (blinking) and also to evacuate within the nasal fossa the tears permanently produced by the lacrimal gland. Some anatomical specifications of the Riolan and Horner muscles are the right technical answer to the problem. The mouth is an orifice bounded by two lips encircled by a complex “star-shaped” muscle allowing it to move in different directions for mimic and phonation. The facial nerve has in fact two parts in its brainstem nucleus corresponding to these two functions, which explains the possibility of complete or incomplete facial palsy.
6.1 Introduction
The facial skin mask has, included in its deep layer, striated muscles so called skinny muscles or platysma, allowing its animation focused on three holes: the two palpebral openings of orbits and the oral orifice.
It is important to note that the facial muscles have no muscle spindles nor proprioceptive corpuscles; as since they are inserted within the skin, it is then the skin sensitivity which provides their functional coordination through the trigeminal nerve (V). Truncal anesthesia performed by a dentist changes the sensitivity of the face that can induce in the patient a fake feeling of paralysis. But the numbness felt is an impression fortunately transient due to a loss of facial expression.
This also explains why the mixed facial nerve (VII) of the second visceral arch becomes a purely motor nerve when it arrives within the face after innerving the stylohyoid muscle and the posterior belly of the digastric muscle (Figs. 6.1, 6.2 and 6.3). We must emphasise the importance for surgeons who can, for facemask resuscitation in case of facial palsy, successfully use a suture with the hypoglossal nerve (XII), a purely motor one [1–7]. Sutures between nerves of identical structure (purely sensory or purely motor) have always better clinical outcomes.
Fig. 6.1
The extra-temporal facial nerve (VII). (a) Sagittal section of a right ear seen from outside: 1. Cerebellum; 2. Posterior semi-circular canal (SCC); 3. Lateral SCC; 4. Incudo-mallear joint; 5. Descending portion of the facial canal; 6. TM joint; 7. Parotid gland; 8. Facial nerve emerging at the stylomastoid foramen. (b) Serial section inside the previous one: 1. Facial nerve elbow below the lateral SCC; 2. Chorda tympani between malleus and incus; 3. Facial nerve and stylomastoid artery; 4. Mastoid process; 5. Fibrovenous retrocondylar pad; 6. Meniscus of TMJ. (c) Sagittal section at the level of utricular macula: 1. Superior SCC; 2. Posterior SCC; 3. Utricular macula; 4. Tympanal bone (vaginal crest); 5. Digastric posterior belly nerve. (d) Sagittal section 3 mm inside the previous section: 1. Geniculate ganglion; 2. Stapes; 3. Facial nerve entering within the parotid gland; 4. Digastric posterior belly; 5. Parotid gland
Fig. 6.2
The intrapetrous facial nerve. (a) Serial anatomical sagittal sections of a right ear (from outside to inside): 1. Facial nerve in its tympanic portion; 2. Tympanic membrane; 3. Posterior canal of chorda tympani. (b) Successive section: 1. Stapedial muscle; 2. Stapes and its tendon; 3. Facial nerve in its descending mastoid canal. (c) Successive section: 1. Lateral SCC; 2. Elbow of facial nerve; 3. Chorda tympani. (d) Successive section: 1. Lateral SCC; 2. Anterior process of malleus; 3. Meniscus of TM joint. (e). Axial section of a right ear seen from above: 1. Vestibular nerve; 2. Cochlear nerve; 3. Cochlea; 4. Facial nerve in its tympanic portion; 5. Incudo-mallear joint; 6. Vestibule and utricular macula; 7. Endolymphatic canal and its ampulla
Fig. 6.3
Serial anatomical coronal sections of a right ear seen from the front. (a) Note that the roof of the meatus acusticus externus is at the same level as the floor of the meatus acusticus internus: 1. Cartilage of the meatus acusticus externus; 2. Parotid salivary gland; 3. Posterior part of the meatus acusticus externus with tympanic membrane; 4. Cochleo-vestibular complex; 5. Internal carotid artery; 6. Meatus acusticus internus; 7. Trigeminal nerve (V). (b) Details of the cochleo-vestibular complex: 1. Superior SCC; 2. Ampulla of the lateral semicircular canal; 3. Facial nerve; 4. Posterior branch of stapes; 5. Oval window with plate of stapes; 6. Vestibular nerve (VIII); 7. Facial nerve; 8. Labyrinthic artery; 9. Cochlear nerve; 10. Spiral lamina of the first roll of cochlea. (c) Posterior section: 1. Vestibule with lateral and superior SCC; 2. Temporal lobe of brain; 3. Tentorium cerebelli; 4. Pons; 5. VII and VIII nerves. (d) Posterior section: 1. Mastoid process; 2. Facial nerve in its descending canal; 3. Posterior part of the lateral and superior SCC; 4. Jugular vein bulb; 5. Facial nerve within the groove formed by cochleovestibular nerve (VIII)
6.2 Facial Superior Orbital Floor
The first major technical problem of the construction of the facial superior orbital floor is the cornea, the window of the eye and also, by its two radius of curvature, the first dioptre of the optical system. It must necessarily be perfectly translucent and transparent to ensure excellent image capture. This excludes for its trophicity the presence of a vascular network disrupting the vision significantly.
6.2.1 Layers of the Cornea
It has several layers from surface to depth:
a stratified squamous epithelium with three cell seats: superficial, intermediate and basal;
a nerve plexus layer dependent of the ophthalmic branch of the trigeminal nerve (V1) and limited by the anterior limiting lamina of Bowman;
a connective tissue stroma rich in mucopolysaccharides in which are keratocytes;
the limiting posterior of Descemet;
finally a pavement endothelium.
It continues directly with the sclera at the limbus level and with the bulbar conjunctiva. Its thickness is variable and in the order of 1 mm or more.
6.2.2 Trophicity of the Cornea
Technically, it is necessary to ensure the trophicity of this avascular membrane by maintaining a fluid trophic space on both sides:
on the internal side, the anterior chamber with a large space is located in front of the lens and iris diaphragm. The aqueous humour is placed inside. It is a transparent liquid, similar to cerebrospinal fluid, secreted continuously by the ciliary processes of the ciliary body and continuously absorbed by the venous sinus of the sclera containing the fine Schlemm’s canal at the iridocorneal angle. The lack of absorption of liquid by pinching the angle and reduction in size of the canal creates glaucoma, phenomenon of hypertension within the inextensible eyeball causing, as in brain hydraulic disorders, nerve damage in the retina, a major cause of blindness in adults;
on the external side, the solution is more complex because the cornea, looking to the outside world, is exposed to air. It is therefore necessary to maintain a fluid film on the corneal surface without disrupting vision. This is done by the extraordinary lacrimal system that of course does not exist in fishes that are totally and permanently immersed in water.
It consists of a gland logically placed at the upper side corner of the orbit, the lacrimal fluid flowing in the superior fornix of the bulbar conjunctiva. The permanent secretion is controlled by post-ganglionic parasympathetic fibres of the pterygopalatine ganglion activated by the facial nerve and conveyed by the lacrimal branch of the trigeminal nerve, which receives the vegetative communicating branch of the zygomatic nerve on the side wall of the orbit (Figs. 6.4 and 6.5).
