The face is a complex biological structure. The
group found it
interesting to attempt to create an exhaustive list of the physical
structures of the face and their role in producing facial expressions.
The overall shape of the face is determined by the underlying bone shapes of the skull and the mandible (jaw bone). The bones are generally considered to be rigid in most applications of facial modeling, however it is obvious that changes in shape must be accounted for in any application concerning modeling of children or of the growth process. From a physical point of view, it is also commonly noted in the medical community that soft tissue always shapes hard tissue - that is to say that if bone is compressed by muscle actions, the bone will eventually be reshaped in response.
The medical term ``joint'' refers to any region where two distinct bones come together. Several bone masses make up the skull, but by adulthood they have fused together to the extent that the jaw is the only feature of the face which fits our common sense definition of a joint as seen in other parts of the body. The jaw is referred to as the temporo-mandibular joint (TMJ). To a first approximation, the TMJ can be treated as a hinge joint. However, in practice it is important that the muscles control the lower jaw in all six degrees of freedom (this is particularly useful for producing grinding actions in chewing).
Several layers of soft tissue cover the bones of the face. Although the tissues can be categorized by function and material content, in vivo the difference between layers of tissue is less distinct (in any given volume of tissue, there may be muscle fibers interspersed with the collagen network of the dermis).
The muscles of facial expression tend to be of the flat, diffuse variety-more like the smooth muscles of the gut than the cylindrical muscles used for locomotion and manipulation in the arms and legs. Whereas the cylindrical muscles have well defined origin and insertion points, the muscles of facial expression have broad attachment areas integrated in the tissue. There may be several layers of muscle fibers connected to the same part of the anatomy (for instance the levator labii and the risorius muscles both insert at the corner of the mouth and are involved in raising it, but they differ in origin). Such muscles may or may not always be independently controllable.
The mechanical behavior, particularly the Poisson effect and the
elasticity, of the skin and soft tissue is one of the primary
determinants of the change of appearance with facial expressions. The
Poisson effect describes the tendency of the material to preserve its
volume when changing length. Since much of the mass in the soft
tissue is water, the soft tissue is nearly incompressible. Thus when
muscles cause a contraction along one axis, the face must bulge along
another; since the underlying hard tissue forms a firm foundation,
facial actions almost always cause the skin to bulge out from the
face. This change in the surface becomes visible through changes in
the silhouette edge of the face and through changes in the surface
shading of the face. The other major mechanical effect, elasticity,
is visible in expression through the displacement of features. When a
muscle causes a movement at a particular point of the face (say the
corner of the lip is raised), the tissue in the surrounding area is
displaced also. The amount of displacement of a particular point is
determined by its distance from the point being moved, the elasticity
of surrounding tissue, and the influence of boundary conditions (such
as a rigid attachment to hard tissue). In general, the Poisson effect
and the elasticity of the soft tissue (represented mathematically by
Poisson's ratio () and Young's modulus (E) will be different
depending on the material being examined. They also may depend on the
orientation of motion with respect to, for example, the underlying
orientation of fibers of the tissue. Therefore, these values should
be considered to be multiple valued functions of spatial location.
The detailed response of the facial soft tissue to muscle action is determined by the distribution of types of material and the orientation of the fibers. In the absence of physical trauma or surgery, these conditions are determined by growth and aging processes. Obviously, the general shape of the face and the locations of facial features are determined by the developmental process. For an individual, there will be natural areas where a crease in the skin occurs, such as at a naso-labial fold. These locations are characterized physically as areas where the fibrous structure in the tissue is preferentially aligned along the axis of the fold. Similar asymmetric alignments of fibers may arise over time due to the mechanical breakdown of the tissue: age lines and wrinkles. These features of the face occur along lines that are repeatedly exercised during facial activities. The process of wrinkle formation is similar to the fatiguing process in metals and other materials. Scars are characterized by a denser fiber structure and asymmetric fiber alignment.