The face should be considered as a three-dimensional structure with five unique layers. Running from superficial to deep, these layers are:
The outermost layer of the face is the skin, which comprises of the epidermis and the dermis. Beneath this is the subcutaneous layer, where the superficial fat pads are found. The fat pads lie on top of the SMAS; it is important to note that the superficial fat pads are firmly attached to the skin but loosely attached to the SMAS. As a result, these fat pads contribute to pseudoptosis of the skin as they slide along the SMAS layer in the aging process.
Underneath the SMAS is a layer that has traditionally been considered to be loose areola tissue. We now understand that this deeper layer comprises of multiple discrete fat compartments (referred to as the deep fat pads) and facial muscles. Beneath this is the periosteum.
Secretion – The skin secretes sebum from the underlying sebaceous glands. This natural oil helps to keep the skin supple.
Heat Regulation – The body temperature is regulated through the skin. Sweating helps to cool the skin, while erect hair follicles trap a layer of air to warm the body up.
Absorption – Substances can be absorbed through the skin which can be transported into the blood stream.
Protection – The skin acts as a protective barrier against germs and bacteria. The skin also contains melanocytes which produce melanin as protection against UV radiation.
Excretion – The skin contains sweat glands which help to excrete excess waste and toxins out of the body.
Sensation – The skin contains thousands of nerve endings which act as sensors for pain, heat or cold.
Vitamins – The skin helps make Vitamin D which is created by a chemical reaction to sunlight.
A unique characteristic of the epidermis is its ability to regenerate tissue continuously. This process of shedding and renewing of epidermal cells is called desquamation.
From superficial to deep, the layers of the epidermis are:
Stratum Corneum:The outer layer of skin. This layer is the thickest of the epidermal layers and is exposed to the outer elements. The cells in this layer are dry and at. This layer may have between 18-23 layers of at dry cells that are cemented together by lipids, peptides, sebum and ceramides.
Stratum Lucidum: Thickness may vary from 0.5 to 0.8mm on the palms and soles of the feet and can be less than 0.1mm on the eyelids.
Stratum Granulosum: In this layer the lipids separate from the keratin (a non-living substance), ands cells lose a considerable amount of fat and moisture. These cells are approximately 80% keratin and less than 20% water.
Stratum Spinosum: This layer is called the spiny cell layer due to the spiky appearance of the cells. Stratum Germinativum: The basal layer is the only living layer of the epidermis where mitosis takes place. This layer of skin does not have any blood vessels in it. Melanin is found in this layer
The dermis is divided into:
This layer is found directly under the skin. It is rmly attached to the skin but loosely attached to the SMAS, hence volume loss and repositioning of the superficial fat pads result in pseudoptosis of the skin. This layer is divided into separate compartments as follow:
Muscles are classified into three different types:
For the purpose of this course, we are going to concentrate on skeletal muscle, as smooth muscle is mainly found within hollow organs and cardiac muscles are found within the heart.
Skeletal muscles are attached to bones and their main function is movement. These muscles are made up of fine thread-like fibres containing light and dark bands. Skeletal muscles can be made to contract or relax by voluntary will. They have striations due to the actin and myosin fibres and create movement when contracted. There are over 650 different types of muscles in the human body, making up nearly half of the body weight.
Muscles have the following properties:
Excitability – the muscle responds to stimuli
Contractibility – the muscle shortens due to a nerve impulse
Extensibility – the muscle can stretch and increase its length by half
Elasticity – the muscle will return to its normal length
Muscles consist mainly of muscle fibres which are held together by fibrous connective tissue, with numerous blood vessels and nerves penetrating through them. The muscle fibres are made up of myofibrils, which get shorter (contract) in response to a nerve impulse.
Each muscle fibre has an individual wrapping of a fine connective tissue called endomysium, which are then wrapped into bundles called fascicule and are covered by the perimysium. This is what forms the muscle belly and has its own covering called the fascia epimysium. The fascia acts as a ‘cling film’ around muscles, giving them support and acts as a pathway for nerves, blood vessels and lymph vessels.
The bundles of fibres within muscles will determine the shape of the muscle. The commonest muscle fibres arrangements are:
Parallel Fibres – these muscles have fibres that run parallel to each other in length and can sometimes be called strap muscles. These muscles have great endurance but may not be that strong due to their length. An example would be the sternocleidomastoid (SCM).
Circular Muscles – these muscles are usually circular in shape and an example would be the orbicularis oculi muscle.
Convergent – this is where the muscle fibres converge to an attachment. These fibres are arranged to allow maximum force and can sometimes cross joints which have a large range of movement such as the pectoralis major.
Pennate – these are made up of short fibres, so the pull is short but also strong although the muscle tires easily.
Fusiform – these are sometimes included within the parallel muscle group and are made up of spindle-shaped fibres. A good example is the biceps brachii as the belly is wider than the origin and the insertion.
Muscles are only able to contract or pull. This means that they have to work in groups. A joint, therefore has to have two or more muscles working together.
As a muscle contracts, the opposing muscle relaxes and vice versa. This is called antagonistic action.
The face contains two main arterial blood supplies. One arising from the external carotid artery system and one arising from the internal carotid artery system.
The external carotid artery gives rise to the facial artery which crosses the mandible, entering the face at the junction between the posterior one third and the anterior two thirds of the body of the mandible. This also corresponds to the anterior border of the masseter muscle and this can be palpated at this point as well as being identified by a facial artery notch that runs inferiorly along the edge of the mandible. This artery runs underneath the SMAS layer and traverses superiorly and medially towards the corner of the lip before giving rise to an inferior labial and superior labial artery and then continuing in the deep nasal labial fat as the nasolabial artery. This continues along the side of the nose, as the angular artery, and gives rise to an alar branch to supply the skin over the ala of the nose and a lateral nasal branch to supply the skin of the lateral nose.
A further branch of the external carotid artery is its terminal branch, the super cial temporal artery which divides into two main branches and supplies the skin of the temporal and forehead area.
The second main supply to the face arises from the internal carotid artery which gives rise via its various branches to the supraorbital and supratrochlear artery.
There are a number of ligaments that run through the face, from deep to superficial. True retaining ligaments run from the periosteum through the SMAS layer to insert into the dermis. These include the orbital retaining ligament, the zygomatic ligament and the mandibular ligament. A number of smaller ligaments are also present which can be described as condensations of fascia which run from the SMAS layer superior to the skin. The role of these multiple myocutaneous ligaments is to allow the transmission of movements of the muscles of facial expressions to the skin.