Primary Lab Objectives:
Locate anatomical landmarks on the surface of the face, in order to understand the pathways of the superficial arteries and nerves.
Remove the skin of the face leaving the eyelids and lips in place. Identify the superficial muscles of facial expression and the parotid gland.
Identify the superficial temporal, posterior auricular, occipital, and facial branches of the external carotid artery, considering blood flow to each region of the face.
Locate the retromandibular, facial, and external jugular veins.
Explore the parotid gland and trace the parotid duct to where it pierces the buccinator muscle. Note any nerves emerging deep to the parotid gland.
Follow the branches of the facial nerve to the muscles of facial expression and consider the effect of damage to branches of the facial nerve.
Find the auriculotemporal, supraorbital, and infraorbital nerves considering which parts of the face are innervated by which with regards to testing for deficits.
1. SURFACE ANATOMY
- On the face, roughly trace the sensory distribution of the three branches of the trigeminal nerve. Consider how you could test for deficits in this important cranial nerve using patterns of facial innervation.
HINT: When dissecting the superficial layers of the face and scalp, you will encounter sensory branches coming from the trigeminal nerve, and motor branches coming from the facial nerve. You should keep these discrete in your mind, but you do NOT need to distinguish them in your dissection.
The skin of the scalp and face is innervated partly by cutaneous sensory branches of the cervical spinal nerves, but mostly by the fifth cranial nerve (or trigeminal nerve). The trigeminal has three divisions; the ophthalmic (V1), the maxillary (V2) and the mandibular (V3). The V indicates the fifth cranial nerve, and the numbers 1-3 indicate the three divisions. Although the facial nerve (canial nerve VII) also radiates across the face (from beneath the parotid gland), it does NOT provide sensory innervation to the face. Rather, the facial nerve provides motor innervation to the muscles of facial expression.
The subcutaneous tissue of the scalp is densely fibrous and bound tightly both to the overlying skin and to the underlying epicranial aponeurosis (the fascia of the muscle called the occipitofrontalis; described in more detail below). This firm attachment helps anchor the arteries and nerves that invest the scalp. There is limited movement between skin and aponeurosis. When the scalp does slide over the skull it is due to the very loose connective tissue under the epicranial aponeurosis in the subaponeurotic space. When you peel back the scalp for dissection you will, like in brain surgery, expose the subaponeurotic space and pericranium. Fine hairs grow from the superficial layer of the skin, which is accordingly rich in oil-secreting sebaceous glands. The principal arteries of the scalp – the superficial temporal, posterior auricular, and occipital arteries - are branches of the external carotid artery, while the supraorbital and supratrochlear are branches of the ophthalmic arter, a branch of the internal carotid artery
The sensory nerves of the scalp are derived from all three branches of the trigeminal nerve (covering much of the anterior and half of the superior portions of the scalp) and from cervical spinal nerves C.2 and C.3 (covering the rest).
Deep to the vessels and nerves in the superficial fascia of the scalp is a thin fibromuscular sheet known as the occipitofrontalis muscle. It is one of the muscles of facial expression innervated by cranial nerve (CN) VII (aka facial nerve). The portion over the forehead is called the frontalis muscle (frontal belly of the occipitofrontalis), and that over the back of the head is called the occipitalis muscle (occipital belly of the occipitofrontalis). In between the two muscle bellies is a fibrous epicranial aponeurosis (galae aponeurosis). This fibromuscular sheet slides back and forth easily over the skull (but is bound to the scalp thus lifting your forehead skin) when the muscles contract. The frontal belly (the frontalis muscle) raises the eyebrows.
2. DISSECTION OF THE FACE
- Make a midline incision on the cadaver from the point of the chin back to the occiput, leaving the lips and a few millimeters above and below them intact.
- Make incisions encircling the lips and eyes – you will skin these regions later.
HINT: When making these incisions be careful not to cut too deeply so that you keep the epicranial aponeurosis intact for now (you will reflect it later).
- Reflect the skin of the face and scalp laterally all the way down the side of the head making sure to leave the ears in place.
HINT: Since the skin and the superficial fascia are hard to separate from the underlying facial muscles, different dissectors should work on each side, in the hope that any structures damaged on one side will be preserved on the other.
d. After removing the skull and facial skin, identify the occipitofrontalis muscle with its frontal and occipital bellies connected by an aponeurotic tendon.
e. Identify the temporalis muscle fanning out onto the parietal bones of the skull.
f. Identify the masseter muscle running from the zygomatic arch to the mandible and the fat pad immediately anterior to it (and convering the buccinator muscle).
g. Identify the orbicularis oris and orbicularis oculi muscles surrounding the mouth and eye, respectively.
h. Identify the zygomaticus major muscle and smaller elevators and depressors of the mouth (they may be hard to see as distinct bundles).
i. Identify the large salivary gland in front of the ear. This is the parotid gland. It has a duct that passes across the masseter, into the fat pad over the buccinator, and then through the buccinator and into the mouth. The parotid gland will be an important landmark throughout all of today's dissection (you will dissect this structure and its duct in detail in section 5). Many nerves (including branches of the facial nerve) will emerge from underneath the parotid gland and several arteries will travel underneath on their way to the occipital, temporal, and parietal regions of the skull.
3. BRANCHES OF THE EXTERNAL CAROTID ARTERY
Four principal arteries, all of them branches of the external carotid, supply the structures of the face and scalp: the superficial temporal, posterior auricular, occipital, and facial arteries. All the arteries supplying the face anastomose with each other (and with the facial branches of the ophthalmic and maxillary arteries) quite freely.
- Locate the superficial temporal artery on the side (temple) of the head, running up in front of the ear
The superficial temporal artery is the vessel from which the temporal pulse can be taken.
- Just behind the ear, identify the posterior auricular artery.
- Behind the posterior auricular artery, identify the occipital artery.
- Look for the facial artery hooking around the inferior border of the mandible in front of the masseter muscle, pursuing a path towards the corner of the mouth.
- Accompanying the facial artery is the facial vein.
4. VENOUS DRAINAGE
NOTE: The veins of the face are more variable than the arteries, and also anastomose freely with each other. The most common pattern is that shown in the atlas, in which the facial vein flows chiefly into the internal jugular, and the retromandibular vein sends blood to both internal and external jugular channels.
- Trace the drainage of the superficial temporal vein into the retromandibular vein.
- Examine the pathway of the facial vein; note that it takes a course parallel to, but straighter than, that of the facial artery.
- Study the courses that the internal and external jugular veins take through the neck.
5. THE PAROTID GLAND
- Locate the parotid gland just in front of the ear.
- Identify the great auricular nerve crossing the surface of the parotid gland (do not confuse these cutaneous sensory fibers with those of the facial nerve running within the substance of the parotid gland).
- Remove the cutaneous sensory fibers of the great auricular nerve.
- Find the parotid duct crossing the masseter muscle.
- Trace the duct forward to where it pierces the buccal fat pad and the buccinator muscle (a facial muscle) to enter the mouth.
- Clean away the buccal fat pad lying between the masseter and the buccinator muscles to expose the terminal end of the duct.
6. FACIAL NERVE and MUSCLES OF FACIAL EXPRESSION
The motor branch of the facial nerve (VII) traverses the bony capsule of the inner ear and exits the skull through the stylomastoid foramen just behind the external auditory meatus. It runs downward into the parotid gland, and its branches spread out from under the gland's margin in all possible directions:
- upward (temporal branches) to the facial muscles in the forehead and around the eyelids,
- forward (zygomatic and buccal branches) to the muscles around the mouth,
- downward (mandibular and cervical branches) to facial muscles below the mouth, and
- backward (posterior auricular branches) to the trivial muscles that attach to the ear
- Locate as many branches of the facial nerve as you can.
- Trace those fibers that you find to the frontalis, orbicularis oculi, buccinator, orbicularis oris, zygomaticus major and minor, and depressor anguli oris muscles.
- Follow any branches you can find going into the platysma muscle.
7. THE AURICULOTEMPORAL, SUPRAORBITAL, AND INFRAORBITAL NERVES
- Complete your dissection of the scalp by reflecting the epicranial aponeurosis to display the underlying cranial periosteum.
- Note how easy the dissection is in the areolar layer.
- Locate the auriculotemporal nerve accompanying the superficial temporal artery, just in front of the ear.
- Locate the infraorbital branches of V2 emerging onto the cheek just below the orbit - the main trunk will be covered by the facial muscles responsible for raising the upper lip.
- On one side, reflect these facial muscle fibers to expose the trunk of the nerve exiting the infraorbital foramen.
- Locate the fibers of the supraorbital nerve above the orbit, running up over the frontalis muscle.
The cutaneous branches of the trigeminal nerve correspond closely to the primary distributions of those nerves: Vl (ophthalmic) to the orbit, V2 (maxillary) to the upper jaw, and V3 (mandibular) to the lower jaw. The mandibular nerve (V3), which is also the motor nerve to the muscles of mastication, sends cutaneous branches to the skin covering those muscles. The branch overlying the temporalis muscle is the auriculotemporal nerve, and is important because it carries the parasympathetic fibers from the otic ganglion to the parotid gland.
Head wounds and the “dangerous area” of the scalp: There are five levels to the scalp. You can remember them by their mnemonic: S: skin, C: connective tissue , A: aponeurosis of the frontalis and occipitalis muscles (also known as the epicranial aponeurosis or the galea aponeurotica), L: loose subaponeurotic areolar tissue containing the emissary veins and allowing free movement of layers 1 to 3 as a unit, and P: pericranium. Infection can spread easily within the layers and in some cases can lead to serious consequences. This is especially true for the so-called "dangerous area" (the loose aponeurotic layer) because infection can spread into intracranial structures by way of the emissary veins.
Parotid gland malignancy and facial nerve damage: Parotid tumors are rare but can have profound effects on control of muscles of facial expression because swelling in the gland can compress the facial nerve, which emerges from the stylomastoid foramen deep to the parotid gland. Tumors of the parotid gland are often not easily diagnosed until late stages of development, but recognition of the damage they can cause can improve diagnosis. In most cases there is swelling but it is not always obvious or painful. At later stages of development tumors of the parotid, especially malignant ones, involve the facial nerve leading to weakness in muscles of facial expression and symptoms like that of Bell’s palsy. Even in cases where the gland itself does not compress the nerve, it is important to be aware of the relationship between the parotid gland and the facial nerve. Recognize that a surgeon can damage branches of the facial during resection. Additionally, radiation therapy can damage the nerve.